U.  S.  DEPARTMENT  OF  AGRICULTURE. 
BUREAU  OF  ENTOMOLOGY — BULLETIN  No.  90. 

L  O.  HOWARD.  EntomolojM  and  Chief  of  Biimii. 


HYDROCYANIC-ACID  GAS  ITMIGATIOX 
IN  CALIFORNIA. 


I.  FUMIGATION  OF  CITRUS  TREES. 

By  R.  S.  WOGLUM.  M.  S.  A  ,  Special  Field  Agev' 

II.  THE  VALUE  OF  SODIUM  CYAN  ID  FOR  FUMIGATION  PURPOSES. 

By  R.  S.  WOGLUM,  M.  S.  A.,  Special  Field  Agent. 

III.  CHEMISTRY  OF  FUMIGATION  WITH  HYDROCYANIC- ACID  GAS. 

By  C.  C.  McDOXNELL,  Chief,  Insecticide  and  Fungicide  Laboratory. 
Miscellaneous  Division,  Bureau  of  Chemistry. 


WASHINGTON: 
GOVERNMENT  PRINTING  OFFICE. 
1912. 


U.  S.  DEPARTMENT  OE  AGRICULTURE, 
BUREAU  OF  ENTOMOLOGY— BULLETIN  No.  90. 

L.  O.  HOWARD.  Entomologist  and  Chief  of  Bureau. 


HYDROCYANIC-ACID  GAS  FUMIGATION 
[N  CAUFORN1  \. 


L  rUWGATION  OF  CITRUS  TBEE8. 

By  R.  B.  WOGLUM,  M.  S.  A..  Special  Fit  Id  Agent. 

II,  THE  VALUE  01  snnini  CYAND)  FOB  PUMIGATTOW  PURfOSKL 

By  R.  S.  WOGLUM,  M.  S.  A.,  Sptcml  FieM  iffl* 

HI.  CHEMISTRY  Of  FUMIGATION  WITH  BYDROCYANIC-ACID  GAS. 

By  C.  (\  McDONNELL,  Chief,  Insecticide  and  Fungicide  Laboratory, 
Miscellaneous  Division,  Bureau  of  Chemistry. 


WASHINGTON: 
GOVERNMENT  PRINTING  OFFICE. 
1912. 


B  UREA  U  OF  ENTOMOLOGY. 


L.  0.  Howard,  Entomologist  and  Chief  of  Bureau. 
C.  L.  Marlatt,  Entomologist  and  Acting  Chief  in  Absence  of  Chief. 
R.  S.  Clifton,  Executive  Assistant. 
W.  F.  Tastet,  Chief  Clerk. 

F.  H.  Chittenden,  in  charge  of  truck  crop  and  stored  product  insect  investigations. 

A.  D.  Hopkins,  in  charge  of forest  insect  investigations. 

W.  D.  Hunter,  in  charge  of  southern  field  crop  insect  investigations. 

F.  M.  Webster,  in  charge  of  cereal  and  forage  insect  investigations. 

A.  L.  Quaintance,  in  charge  of  deciduous  fruit  insect  investigations. 

E.  F.  Phillips,  in  charge  of  bee  culture. 

D.  M.  Rogers,  in  charge  of  preventing  spread  of  moths,  field  work. 
Rolla  P.  Currie,  in  charge  of  editorial  ^ork. 
Mabel  Colcord,  in  charge  of  library. 

Citrus  Fruit  Insect  Investigations. 

C.  L.  Marlatt,  in  charge. 

R.  S.  Woglum,  W.  W.  Y others,  E.  R.  Sasscer,  J.  R.  Horton,  P.  H.  Timberlake, 

C.  E.  Pemberton,  H.  L.  Sanford,  entomological  assistants. 
J.  G.  Sanders,  collaborator. 
H 


LETTER  OF  TRANSMITTAL 


(Jutted  States  Department  of  Agriculture , 

Bureau  of  Entomouhiy. 
Washington.  P.  01,  September  11,  1912. 
Sir:  I  have  the  honor  to  transmit  herewith,  for  puhlication  as 
Bulletin  Xo.  00  of  tins  bureau,  three  papers  comprising  a  report  on 
an  investigation  of  hydrocyanic-arid  gas  fumigation  of  citrus  orchards 
in  southern  California.  The  preliminary  report  on  the  subject  was 
published  as  Bulletin  Xo.  7\>  of  this  bureau  under  the  title  "Fumi- 
gation Investigations  in  California."  The  present  final  report  is 
divided  into  three  parts,  which  were  published  separately  on  May  10 
and  13,  1011:  [,  entitled  "Fumigation  of  Citrus  Trees,"  by  R.  S. 
Woglum.  a  special  field  agent  of  this  bureau,  containing  the  main 
report  on  the  field  investigations  and  discussing  the  various  details 
of  fumigation  procedure;  II.  a  paper  by  Mr.  Woglum.  "The  Value  of 
Sodium  Cyanid  for  Fumigation  Purposes";  and,  III,  one  by  Mr.  C.  C. 
McDonnell,  Chief  of  the  Insecticide  and  Fungicide  Laboratory, 
Bureau  of  Chemistry,  "Chemistry  of  Fumigation  with  Hydrocyanic- 
acid  Gas." 

Respectfully,  L.  O.  Howard, 

Entomologist  and  Ch  ief  of  Bureau. 

Hon.  James  Wilson, 

Semtarg  of  Agriculturt  . 

in 


Digitized  by  the  Internet  Archive 

in  2013 

h  ttp  ://arc  h  i  ve .  o  rg/d  etai  Is/hy  d  rocy  00  u  n  it 


PREFACE 


An  investigation  into  the  methods  of  fumigating  citrus  trees  with 
hydrocyanic-acid  gas  was  commenced  by  the  Bureau  of  Entomology, 
United  States  Department  of  Agriculture,  during  the  summer  of 
1907,  and  for  a  period  of  three  years  has  been  carried  on  in  California 
by  the  writer  under  the  direction  of  Mr.  C.  L.  Marlatt.  assistant  chief 
of  the  bureau.  This  work  was  undertaken  in  response  to  urgent 
requests  from  the  horticultural  commissions  of  the  principal  citrus- 
fruit-producing  counties  of  southern  California  and  of  many  active 
fruit  growers.  Prominent  in  this  movement  was  Mr.  J,  W.  Jeffrey, 
former  secretary  of  the  Los  Angeles  County  horticultural  Commission, 
and  now  State  commissioner  of  horticulture — a  man  entirely  familiar 
with  the  unsettled  condition  of  fumigation  practice  at  that  time  and 
with  the  need  of  placing  it  on  a  more  scientific  basis.  At  the  com- 
mencement .  the  wcitcr  spent  from  t  hree  to  four  months  in  a.  thorough 
field  investigation  to  acquaint  himself  with  the  conditions  of  citrus 
culture  throughout  southern  California,  the  distribution  of  the  dif- 
ferent citrus  pests  and  the  damage  caused  by  them,  the  existing 
methods  for  their  control,  and  the  methods  of  fumigation  practiced 
in  the  various  citrus  districts. 

During  the  early  part  of  November,   1907,  active  experimental 

held  work  was  commenced  <*>t  Orange,  Cal..  using  an  outfit  belonging 
to  this  bureau,  consisting  of  four  tents  and  the  other  paraphernalia 

necessary  for  practical  fumigation.  Kield  work  of  this  character 
has  been  continued  throughout .  it  being  the  writer's  effort  to  conduct 
the  investigation  on  as  nearly  a  commercial  basis  as  possible  so  that 
the  conditions  and  results  would  be  those  normal  to  the  ordinary 
care  of  citrus  groves.  During  the  work  there  have  arisen  many 
problems  of  a  laboratory  nature",  the  solution  of  which  would  have, 
been  most  interesting,  but  these  problems  for  the  most  part  have 
been  set  aside  except  in  those  cases  where  they  had  a  direct  economic 
bearing  on  practical  work  in  the  held. 

The  results  of  this  investigation  have  very  little  of  the  nature  of 
original  discoveries,  although  there  has  been  acquired  a  vast  amount 
of  exact  information  never  before1  thoroughly  understood.  The 
advance  is  largely  the  result  of  correcting,  correlating,  systematizing, 
and  placing  upon  a  more  scientific  as  well  as  a  more  practical  basis 
methods  which  had  been  practiced  in  California  or  elsewhere  for 
many  years. 

v 


VT 


HYDROCYANIC- ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


All  information,  as  acquired,  of  direct  bearing  on  the  fumigation 
practice  has  been  given  freely  to  the  public  as  soon  as  its  economic 
value  was  established,  largely  by  means  of  addresses,  demonstrations, 
and  printed  reports. 

In  the  present  bulletin  an  attempt  has  been  made  to  present  a 
succinct  account  of  the  completed  results  of  this  fumigation  investi- 
gation as  well  as  a  brief  treatment  of  the  salient  features  of  fumi- 
gation as  practiced  in  California  at  the  present  time.  It  is  of  the 
nature  of  a  handbook  on  the  most  up-to-date  equipment,  methods, 
and  directions  in  orchard  fumigation.  Full  advantage  has  been 
taken  of  the  results  of  other  investigators  in  fumigation;  yet  in  such 
cases  due  credit  is  given  to  the  proper  source.  The  information 
given  in  Bulletin  79  of  this  bureau,  which  is  a  preliminary  report  on 
this  investigation,  has  been  largely  included  in  the  present  bulletin 
in  summarized  form. 

The  writer  desires  to  acknowledge  his  indebtedness  to  the  many 
people  who  have  assisted  him  during  this  investigation  and  facilitated 
the  progress  which  has  been  made.  To  Mr.  C.  L.  Marlatt,  Assistant 
Chief  of  the  Bureau  of  Entomology,  he  is  especially  indebted  for 
valuable  assistance  and  advice.  Acknowledgment  is  also  due  to 
Mr.  Frederick  Maskew,  who  most  capably  assisted  him  in  the  per- 
formance of  many  of  his  experiments  during  the  period  from  Decem- 
ber, 1907,  to  August,  1909.  Valuable  assistance  was  rendered  by 
Mr.  E.  It.  Sasscer  during  the  months  of  August,  September,  and 
October,  1909.  Mr.  W.  W.  Yothers  was  engaged  in  the  work  during 
November,  1909.  To  the  Hon.  J.  W.  Jeffrey,  State  commissioner  of 
horticulture  of  California,  credit  is  due  not  only  for  his  activity  in 
paving  the  way  for  this  investigation,  but  also  for  the  able  support 
given  by  him  since  field  work  was  commenced.  To  Mr.  William  Wood, 
of  Whittier,  Cal.,  the  writer  acknowledges  his  indebtedness  for 
assistance  in  introducing  the  improved  system  of  fumigation  in  the 
region  adjacent  to  Whittier,  as  well  as  for  practical  advice  with  regard 
to  citrus  insects  and  their  control,  a  subject  about  which  Mr.  Wood 
is  especially  well  informed.  This  occasion  is  also  taken  to  thank  the 
various  horticultural  officers  of  southern  California,  packing-house 
managers,  and  the  many  citrus  growers  who  have  assisted  and  sup- 
ported this  investigation. 

The  cooperation  of  the  Bureau  of  Chemistry  of  the  United  States 
Department  of  Agriculture  has  been  an  important  adjunct  of  this 
investigation,  and  to  Mr.  J.  K.  Haywood,  Chief  of  the  Miscellaneous 
Division  of  that  bureau,  and  his  assistants  the  writer  is  indebted  for 
the  carrying  out  of  all  the  chemical  analyses  and  laboratory  tests  of 
materials  and  products  necessary  <<>  the  working  out  of  the  field 
experiments. 

R.  S.  W. 


CONTENTS 


Page. 


Fumigation  of  citrus  trees  R.  S.  Woglum. .  1 

Historical   1 

Recent  renewal  of  interest  in  fumigation  in  California   2 

Extent  to  which  fumigation  is  practiced  in  California   3 

The  various  systems  of  fumigation   4 

By  contract   4 

By  associations   5 

By  counties   5 

By  private  individuals   5 

Extent  and  character  of  citrus  orchards   6 

Insect  enemies  of  citrus  fruits   7 

The  purple  scale  {Lepidosaphes  beckii  Xewm. )   8 

The  black  scale  (Saissetia  oUm  Bern.)   8 

The  red  scale  (( 'hnjsomphnlus  aurantii  Mask.)   9 

The  yellow  scale  ( <  'Itri/som phulns  ritrin  u.s  Coq .  i   10 

The  mealy-bug  (Pttuthcocctu  eitri  Risso)   10 

Apparatus   10 

Tente   10 

Poles  and  derricks   20 

Th<*  McFadden  machine   21 

Supply  cart  and  supply  wagon   22 

Generating  vessels   24 

General  procedure   24 

Calculating  the  dosage   27 

Securing  the  measurements  around  and  over   28 

The  old  method  of  procedure   30 

An  improved  system  <>f  fumigation   32 

Leakage  of  gas   33 

Dosage  schedule   34 

Procedure   37 

Advantages  under  this  system   37 

Experience  with  this  system   38 

The  chemicals  in  fumigation   40 

Potassium  cyanid  (KCN)   40 

Sulphuric  acid  ( II2SG4)   41 

Water  as  a  factor  in  fumigation   44 

The  most  economical  proportion  of  chemicals  to  use   47 

The  amount  of  chemicals  in  very  small  dosages   48 

Mixing  the  chemicals   48 

Effect  of  the  presence  of  sodium  chlorid  on  the  amount  of  gas  given  off.  49 

Nature  of  the  residue   50 

Dosages  for  various  scale  pests   51 

Factors  which  affect  the  dosage   52 

The  purple  scale   53 

The  red  scale   57 

The  black  scale   59 

The  yellow  scale   GO 

Dosages  in  general  fumigation   61 


i  The  threo  papers  constituting  this  bulletin  were  issued,  the  first  on  May  13,  1911,  and  the  last  two  on 
May  10, 1911. 


VIII         HYDROCYANIC-ACID'  GAS  FUMIGATION  IX  CALIFORNIA. 


Fumigation  of  citrus  trees — Continued.  page. 

Time  of  the  year  for  fumigation   61 

Fumigation  for  the  mealy-bug   63 

Fumigation  during  the  blossoming  period   64 

Fumigation  while  the  fruit  is  of  small  size   65 

Fumigating  lemons   65 

Effects  of  fumigation  on  unhealthy  trees   66 

Greater  susceptibility  to  injury  of  some  varieties  than  others   67 

The  distribution  of  gas  within  a  tent   67 

Fumigation  for  physiological  effects   67 

Effects  of  meteorological  elements  on  fumigation   68 

Injury  to  sprayed  trees   72 

The  appearance  of  fumigated  trees   73 

The  presence  of  old  scales  on  fumigated  trees   74 

A  device  for  covering  fumigation  generators   74 

The  effect  of  climatic  conditions  on  scale  insects   76 

The  effect  of  fumigation  on  ladybirds  (Coccinellidae)  and  Scutellista  cyanea 

Motsch   §?  77 

The  cost  of  fumigation   -78 

General  cautions   80 

List  of  the  writer's  published  articles  and  addresses  on  the  fumigation 

investigation  in  California  '   81 

The  value  of  sodium  cyanid  for  fumigation  purposes  R.  S.  Woglum. .  83 

Introduction   83 

Strength  of  sodium  cyanid  expressed  in  terms  of  potassium  cyanid   85 

Proportion  of  chemicals   85 

Field  tests   86 

Action  of  sodium  chlorid   87 

The  kind  of  cyanid  to  purchase   88 

Dosages  with  sodium  cyanid   88 

Dosages  recommended  for  scale  pests   89 

Comparison  of  sodium  cyanid  and  potassium  cyanid  for  general  fumigation.  90 

Chemistry  of  fumigation  with  hydrocyanic-acid  gas  C.  C.  McDonnell. .  91 

Introduction   91 

I.  Analyses  of  chemicals  used  for  the  production  of  hydrocyanic-acid  gas.  91 

Sulphuric  acid   91 

Cyanid  samples   92 

II.  Proportion  of  cyanid,  sulphuric  acid,  and  water  for  best  yield  of  gas. .  93 

Potassium  cyanid   93 

Sodium  cyanid   93 

III.  Action  of  mineral  acids  on  cyanids  and  hydrocyanic  acid   96 

Action  of  sulphuric  acid  on  hydrocyanic  acid   97 

Action  of  hydrochloric  acid  on  hydrocyanic  acid   98 

Effect  of  the  presence  of  sodium  chlorid  in  cyanids  on  the  yield  of  hydro- 
cyanic-acid gas  in  fumigations  •.   99 

Description  of  apparatus   100 

Details  of  manipulation  :   100 

Results  of  experiments   101 

Ammonia  formed  from  the  decomposition  of  the  cyanid   102 

Effect  of  the  pn-sence  of  sodium  nitrate  in  cyanids  on  the  yield  of  hydro- 
cyanic-acid gas   '  103 

Summary   104 

Index   107 


ILLUSTRATION'S. 


PLATES. 

Page. 

Plate  I.  Method  of  covering  small  tree  with  bell  or  hoop  tent   lt> 

II.  Fig.  1. — Brick  furnace,  tank,  and  derrick  used  in  the  tannin  treat- 
ment formildew.  San  Bernardino  County,  (Ad.   Fin.  2. — Machine 

for  covering  trees  with  sheet  tents   20 

III.  Fig.  1. — Method  of  attaching  lent  to  hoisting  pole  by  a  half  hitch  of 
the  rope.  Fig.  2.  -Top  of  derrick,  showing  method  of  attaching 
pulley.  Fig.  3. — Base  of  derrick,  showing  method  of  construct- 
ing braces   22 

JTV.  Fig.  I . — Supply  cart  used  with  the  improved  system  of  fumigation. 
Fig.  2. — Supply  wagon  devised  by  C.  E.  McFadden.  of  Fullerton, 

Cal   22 

V.  Figs.  1-5.— Successive  stages  in  placing  a  tent  over  a  tree  with 
poles.  Fig.  ii.  -A  tented  tree,  showing  method  of  securing  the 
distance  around  the  bottom  of  the  tent  by  means  of  a  tape  at- 
tached to  an  iron  rod   2<> 

VI.  Removing  the  tent  from  one  tree  onto  another  by  means  of  poles. .  2<> 

VII.  Placing  a  sheet  tent  over  a  tree  by  means  of  derricks   2S 

VIII.  Fig.  L. — A  row  of  tented  trees,  with  cart  at  one  end  of  row,  ready  to 

commence  dosing.    Fig.  2.    Dosing  a  tree   38 

IX.  Dosage  Schedule  A.  for  high-grade  sodium  cyanid   88 

X.  Dosage  Schedule  J-A.  for  high-grade  -odium  cyanid   88 

TEXT  FIGURES. 

Fig.  1.  Ma]>  showing  principal  localities  in  southern  California  where  citrus 

fruits  are  produced   7 

2.  Plan  for  construction  of  octagonal  sheet  tent.  50  feel  across,  showing 

lines  used  in  constructing  octagon   14 

3.  Method  of  attaching  hooks  to  tent  when  covering  trees  with  aid  of  der- 

ricks  16 

4.  Ends  of  hoisting  poles  used  in  placing  tents  over  trees   20 

5.  Earthenware  acid  jar  with  attachment!  for  field  use   23 

G.  Ciuboy  with  handles  attached  to  fat -dilate  pouring  the  acid  and  earning 

the  carboy   26 

7.  Outline  of  a  sheet  fumigation  tent  marked  according  to  the  Morrill 

method   29 

8.  Man  carrying  tray  and  water  bucket  as  practiced  under  the  old  system 

of  fumigation   31 

9.  Dosage  schedule  No.  1,  for  potassium  cyanid   34 

10.  Thart  showing  total  amount  of  gas  evolved  when  different  proportions 

of  water  are  used   45 

]  1 .  Dosage  schedule  No.     for  potassium  cyanid   59 

12.  A  cover  device  attached  to  a  fumigation  generator   75 

13.  Laboratory  apparatus  used  in  the  decomposition  of  cyanids  and  collec- 

tion of  the  liberated  hydrocyanic-acid  gas   100 

IX 


U.  S.  D.  A.,  B.  I.  Bui.  90,  Part  L 


Issued  May  13.  1911. 


HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

FUMIGATION  OF  CITRUS  TREES. 

By  11.  s.  W'OGLUM. 
Special  Field  Agent,  Bureau  of  Entomology. 

HISTORICAL.' 

To  Mr.  I).  W,  (oquillett .  of  the  Bureau  of  Entomology,  United 
States  Department  <>t  Agriculture,  belongs  the  credit  of  first  deter- 
mining the  great  value  of  hydrocyanic-acid  gas  for  destroying  insect 
pests  on  plants.  Dining  the  fall  of  lssr>,  while  a  special  agent  of 
what  was  then  the  Division  of  Entomology,  experimenting  upon  the 
cottony-cushion  scale  (Icerya  purclmsi  Mask.)  in  orange  orchards  in 
California,  he  discovered  this  gas  t<>  he  a  most  efficient  insecticide  for 
scale-insect  pests  of  citrus  trees,  ami  his  continued  experimental 
work  placed  its  use  on  such  a  practical  basis  that  by  1X90  it  had  com- 
menced to  be  employed  quite  extensively  in  a  commercial  way. 

The  use  of  this  gas  was  restricted  to  California  until  the  winter  of 
1892-3,  during  which  time  Prof.  EL  A.  Morgan  gave  it  a  trial  on 
orange  t  ices  in  southern  Louisiana.  The  following  year,  1803,  found 
it  on  trial  against  the  San  Jose  scale  in  Virginia  and  against  citrus 
msect  pests  in  Florida,  in  Montserrat.  British  West  Indies,  and  in 
('ape  Colony.  South  Africa.  It-  subsequent  development  and  use 
has  been  rapid  as  well  as  extensive,  so  that  to-day  fumigation  of  citrus 
trees  is  carried  on  in  California.  Florida.  Australia.  Japan,  and  the 
colonies  of  South  Africa.  At  the  time  of  this  writing  the  practice  is 
being  introduced  into  Spain  and  Porto  Rico. 

The  great  success  attending  the  hydrocyanic-acid-gas  treatment 
against  the  scale  pests  of  citrus  trees  soon  brought  about  its  intro- 
duction into  a  broader  held  of  activity.  This  gas  was  given  its  first 
trial  on  deciduous  trees  by  Mr.  D.  W.  Coquillett  in  1894  at  Char- 

i  The  Canadian  Entomologist  for  1S77,  volume  9.  pages  139-140,  contains  mention  of  an  experiment  by 
James  T.  Bell  in  which  an  insect  cabinet  was  freed  from  insect  pests  by  dropping  sulphuric  acid  on 
potassium  cyanid.  This  is  the  first  record,  so  far  found,  of  the  rapid  development  of  the  gas  by  combin- 
ing sulphuric  acid  with  lump  cyanid  with  the  object  of  killing  insects.  The  use  of  cyanid,  however,  as  a 
means  of  killing  insects  in  collectors'  bottles  is  very  old.  The  gas  liberated  from  moistened  lump  cyanid  is 
the  same  as  that  generated  by  the  action  of  sulphuric  acid  or  hydrochloric  acid,  on  the  authority  of  Dr. 
J.  K.  Haywood,  of  the  Bureau  of  Chemistry  of  this  department.  The  action  of  the  acid  merely  hastens 
the  generation  of  the  gas.  It  does  not  seem  desirable  or  appropriate,  therefore,  in  a  discussion  of  the 
broad-scale  economic  use  of  this  gas  for  the  destruction  of  insects  in  orchards  or  in  buildings,  to  consider 
these  much  earlier  and  minor  uses  of  the  gas  by  collectors  for  killing  insects,  or  similar  limited  uses.— 
C.  L.  If. 

1 


2 


HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


lottesville,  Va.  The  same  year  it  was  first  used  in  the  treatment  of 
nursery  stock,  and  its  development  along  this  line  has  been  so  great 
and  important  that  to-day  in  many  States  the  fumigation  of  decidu- 
ous stock  before  it  is  planted  is  required  by  law.  The  use  of  hydro- 
cyanic-acid gas  against  insects  affecting  greenhouse  plants  has  been 
successfully  carried  on  for  a  number  of  years.  Among  the  other 
important  uses  to  which  this  gas  has  been  successfully  put  are  the 
treatment  of  mills,  various  other  buildings,  and  stored  products 
infested  with  insects.  The  ease  with  which  this  gas  may  be  generated 
as  well  as  its  destructive  power,  greater  than  that  of  any  other  known 
insecticide,  leads  the  writer  to  believe  that  as  soon  as  the  various 
uses  to  which  this  gas  may  be  put  have  been  thoroughly  investigated 
and  placed  on  a  stable  basis  the  future  development  of  hydrocyanic- 
acid-gas  fumigation  will  be  quite  as  important  and  extensive  as  has 
been  its  past  development. 

RECENT  RENEWAL  OF  INTEREST  IN  FUMIGATION  IN 
CALIFORNIA. 

The  hydrocyanic-acid-gas  treatment  of  citrus  trees  continued  to 
become  more  widely  used  and  to  hold  general  favor  with  the  fruit 
growers  of  southern  California  until  about  1901,  when  the  distillate 
spray  was  introduced.  The  treatment  of  trees  with  distillate  was 
much  cheaper  than  with  hydrocyanic-acid  gas.  This  fact,  together 
with  the  fact  that  the  distillate  treatment  was  indorsed  by  many  of 
the  more  prominent  horticultural  authorities  and  fruit  growers  led  to 
its  widespread  use  during  the  next  few  years.  Simultaneously  the 
introduction  from  South  Africa  of  Scutellista  cyanea  Motschulsky,  the 
parasite  of  the  black  scale  (Saissetia  olese  Bern.),  and  its  subsequent 
splendid  showing  led  many  people  to  abandon  treating  their  orchards 
in  the  hope  that  this  beneficial  insect  would  hold  the  black  scale  in 
check. 

By  1903-1905  it  had  become  very  evident  that  the  distillate  spray 
had  not  only  failed  to  keep  the  scales  under  control,  but  that  its  con- 
tinued use  in  many  cases  produced  an  injurious  effect  upon  the  tree 
itself.  The  Scutellista  also  had  failed  to  control  the  black  scale, 
although  even  a  conservative  must  admit  that  its  work  has  been  of 
a  most  praiseworthy  type.  Spraying  rapidly  sank  into  disuse  during 
190.")  and  1906,  until  at  the  present  time  it  has  almost  entirely  given 
way  to  fumigation.  The  experience  of  the  prominent  fruit  growers 
wiih  the  distillate  spray  lias  thoroughly  satisfied  them  of  the  great 
superiority  <>f  the  hydrocyanic-acid-gas  treatment  to  that  with  a 
spray  for  scale  insects  on  citrus  trees. 

In  the  winter  of  1  903-4,  Dr.  G.  Harold  Powell,  then  of  the  Bureau  of 
Plant  Industry,  United  States  Department  of  Agriculture,  com- 
menced an  investigation  of  the  decay  of  oranges  while  in  transit 


FUMIGATION  OF  CITRUS  TREES. 


3 


from  California.  His  efforts  resulted  in  determining  that  the  decay 
was  almost  entirely  the  outcome  of  mechanical  injury  to  the  skin  of 
the  fruit  during  its  picking  and  handling  in  the  packing  house'.1 
Oranges  are  washed  primarily  to  remove  the  sooty-mold  fungus  that 
grows  in  the  so-called  honeydew  excreted  by  the  black  scale.  Dr. 
Powell  demon-tinted  that  the  decay  in  washed  fruit  is  much  greater 
than  in  unwashed  fruit.  This  led  the  fruit  growers  to  understand 
that  the  necessity  of  washing  fruit  should  he  avoided  by  controlling 
the  leak  in  the  orchard. 

A-  a  direct  result  of  Dr.  Powell's  investigations,  and  knowing  from 
past  experience  that  the  distillate  spray  and  the  SeuteUista  parasite 
were  inadequate  to  control  the  scale,  fruit  glowers  took  a  renewed 
interest  in  fumigation.  This  Led  to  a  demand  for  an  investigation 
of  thi>  process  to  he  conducted  hy  t  he  Tinted  St  at es  Depart  men t  of  Agri- 
culture, and  the  following  year.  1(.M»7.  the  writer  Was  detailed  to  this 
field.  The  fumigation  practice  was  then  in  a  very  chaotic  condition 
as  the  outgrowth  of  years  of  use  without  any  special  effort  to  have 
the  pmce^  standardised.  Indeed,  it  was  a  favorite  pose  of  many 
professional  fumigators  to  veil  their  operations  in  mystery  in  order 
to  secure  the  reputation  of  being  authorities  in  a  practice  which  they 
made  to  appear  complicated  and  difficult  of  understanding.  Conse- 
quently the  growers,  for  the  most  part,  although  arranging  to  have 
their  orchards  fumigated,  took  no  interest  in  a  procedure  w  hich  they 
little  understood. 

In  the  face  of  this  situation  the  first  reports  of  this  investigation 
given  out  in  100S  attracted  the  immediate  attention  of  the  fruit 
growers.  After  gaining  a  general  understanding  of  the  process  of 
orchard  fumigation  the  growers  in  many  localities  have  become  much 
interested  and  subsequently  have  adopted  or  have  caused  to  be 

adopted  the  more  important  recommendations  of  this  investigation. 

This  adoption  of  better  methods  has  led  to  more  satisfactory  work 
generally.  The  grower  has  immediately  seen  the  advantage  of  better 
methods,  with  the  result  that  where  formerly  many  were  with  diffi- 
culty induced  to  have  their  trees  fumigated,  to-day  the  successful 
orchardist  needs  no  inducement  whatever,  but,  on  the  contrary,  re- 
quires that  his  trees  be  treated  whenever  their  condition  appears  to 
demand  it.  This  public  interest  in  fumigation  has  made  it  one  of 
the  very  live  topics  in  the  horticultural  field  in  southern  California 
to-day. 

EXTENT  TO  WHICH  FUMIGATION  IS  PRACTICED  IN  CALIFORNIA. 

Commercial  fumigation  of  citrus  trees  is  confined  to  six  counties 
of  southern  California,  viz.  Ventura,  Los  Angeles,  Orange,  Riverside, 
San  Bernardino,  and  San  Diego.    In  these  counties  about  85  different 

'  Bui.  123,  Bur.  l'lant  Ind.,  U.  S.  Dept.  Agr..  1908. 


4 


HYbKOCYANIOACID  GAS  FUMIGATION  IN  CALIFORNIA. 


parties,  including  contractors,  associations,  county  horticultural 
commissions,  and  private  individuals,  owned  approximately  5,150 
tents  on  June  1,  1910,  the  date  on  which  the  securing  of  these  data 
was  completed. 

In  order  to  ascertain  the  extent  to  which  fumigation  is  now  prac- 
ticed, as  well  as  the  tax  which  this  procedure  annually  places  on 
citrus  fruit  growers,  a  careful  canvass  of  the  different  parties  operating 
tents  has  been  made.  This  canvass  has  resulted  in  showing  that 
approximately  36,000  acres  were  treated  during  the  year  from  July, 
1909,  to  July,  1910.  Many  fumigators  gave  the  number  of  trees 
which  they  treated;  others  the  acreage  alone.  The  average  orchard 
will  approximate  90  trees  to  the  acre,  and  in  those  cases  in  which 
estimates  were  returned  in  acreage  alone,  this  number  has  been 
considered  to  comprise  an  acre.  Wherever  not  known,  the  cost  of 
fumigating  a  tree  has  been  placed  at  30  cents,  which  price  ap- 
proximates very  closely  the  cost  of  fumigating  the  average-sized 
citrus  tree  in  California.  Calculated  on  this  basis,  the  cost  of  fumiga- 
tion of  the  citrus  orchards  of  southern  California  during  the  season 
1909—1910  approximated  $1,000,000. 

THE  VARIOUS  SYSTEMS  OF  FUMIGATION.  • 

Each  of  the  citrus-fruit-producing  counties  of  southern  California 
has  a  board  of  horticultural  commissioners  consisting  of  three  mem- 
bers whose  duties  are  to  supervise  the  destruction  of  insect  pests, 
plant  diseases,  and  noxious  weeds  within  their  respective  counties. 
In  the  three  greatest  citrus-fruit-producing  counties — Los  Angeles, 
Riverside,  and  San  Bernardino — numerous  inspectors  are  also 
employed  to  assist  in  carrying  on  this  important  work.  As  a  matter 
of  convenience  the  counties  are  usually  divided  into  three  districts, 
each  of  which  is  supervised  by  one  of  the  commissioners.  If  inspectors 
are  employed,  usually  each  is  allotted  a  limited  portion  of  one  of 
these  districts,  and  is  held  responsible  for  the  proper  control  of  pests 
therein.  He  advises  when  the  trees  shall  be  fumigated,  and,  after 
arranging  for  the  execution  of  the  work,  is  supposed  to  see  that  it  is 
properly  carried  out.  There  are  several  different  systems  under 
which  the  work  may  be  done. 

BY  CONTRACT. 

The  larger  part  of  fumigation  is  carried  out  under  the  contract 
system.  Individuals  or  firms  that  possess  complete  equipment  for 
commercial  fumigation  and  practice  fumigation  as  a  business,  enter 
into  an  agreement  with  the  grower,  who  desires  to  have  his  orchard 
treated,  to  do  the  work  for  a  certain  sum.  The  rate  is  seldom  uni- 
form but  varies  with  such  factors  as  the  character  of  the  ground,  the 
acreage,  and  the  size  and  arrangement  of  trees.    Usually  the  cyanid 


FUMIGATION  OF  CITRUS  TREES. 


5 


and  acid  are  furnished  by  the  contractor  at  a  certain  price  per  pound, 
although  sometimes  the  grower  himself  supplies  them.  In  the  latter 
case  the  sole  consideration  is  the  cost  of  covering  per  tree. 

BY  ASSOCIATIONS. 

A  citrus  association  is  composed  of  a  Large  number  of  growers  from 
the  same  district  organized  for  the  purpose  of  cooperation  in  the 
handling  of  their  fruit.  Some  of  these  associations  own  fumigating 
outfits  which  are  utilized  in  the  treatment  of  orchards  belonging  to 
its  members.  The  manager  of  the  association  looks  after  the  pur- 
chasing of  chemicals  and  supplies,  and  also  selects  competent  men  to 
run  the  outfits.  The  inspector  of  the  district  usually  directs  the 
movements  of  the  outfit  from  one  orchard  to  another.  Under  this 
system  the  chemicals  and  labor  are  supplied  at  actual  cost,  plus  a 
slight  allowance  for  the  purchase  as  well  as  wear  and  tear  of  equip- 
ment.   In  short,  this  system  is  supposed  to  be  merely  self-supporting. 

BY   ((U  NTIES. 

Each  of  the  county  boards  of  horticulture  owns  a  greater  or 
smaller  number  of  fumigation  tents.  In  San  Bernardino  County  this 
system  has  reached  its  greatest  development,  for  here  the  horticul- 
tural commission  owns  fully  500  tents  and  carries  on  more  work 
annually  than  all  other  systems  combined.  This  fumigation  is  under 
the  personal  direction  of  a  county  horticultural  ollicer.  The  cost  to 
the  grower  of  treatment  by  these  out  (its  is  usually  what  it  actually 
costs  the  county  to  perform  the  work.  An  important  consideration 
in  favor  of  the  system  of  county  owned  tents  is  that  it  readily  enables 
the  treatment  of  trees  on  city  lots  and  in  small  orchards  in  out-of-the- 
way  places  which  otherwise  would  in  all  probability  be  neglected. 

BY   PRIVATE  INDIVIDUALS. 

Many  citrus  fruit  growers  who  control  a  considerable  acreage  have 
fumigation  outiits  for  their  own  work.  In  a  few  cases  two  or  three 
growers  in  a  locality  combine  in  owning  an  outfit.  The  private 
ownership  of  tents  i>  rapidly  gaining  in  favor  and  well  merits  this 
increased  popularity,  as  it  possesses  decided  advantages. 

Excepting  private  ownership,  it  would  be  scarcely  possible  to  say 
which  of  these  systems  is  superior.  Each  has  its  advantages.  While 
one  system  may  prove  superior  in  one  locality  it  might  prove  less 
successful  in  another.  The  reason  for  success  or  failure  lies  not  in 
the  system  itself  but  largely  in  the  personal  element  directing  and 
conducting  the  procedure.  A  reckless,  uneconomical,  or  unreliable 
director  of  any  one  system  will  achieve  inferior  results  and  give  less 
satisfaction  than  a  careful,  economical,  and  perfectly  reliable  one 


6  HtPROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

under  any  of  the  others.  This  is  mainly  clue  to  the  fact  that  directors 
of  the  former  class  are  likely  to  employ  field  men  of  inferior  qualifi- 
cations. Efficient  fumigation  at  the  present  time  means,  for  the 
most  part,  that  the  men  in  the  field  performing  the  operations  are 
careful,  conscientious,  and  reliable.  Otherwise  the  work  is  likely  to 
be  performed  in  a  slipshod,  hasty  manner,  along  lines  of  least  resist- 
ance. Work  of  this  character,  combined  with  the  element  of  guess- 
work in  deciding  the  dosages  and  proportions  of  chemicals  to  be 
used,  has  been  responsible  for  most  of  the  unsuccessful  results.  If 
perfectly  reliable  men  are  employed  to  carry  on  the  actual  work  in 
the  field,  using  the  most  approved  methods,  success  will  be  as  marked 
with  one  system  as  with  another. 

The  recent  horticultural  ordinances  of  Los  Angeles,  San  Bernardino, 
and  Riverside  Counties  requiring  fumigators  to  be  licensed  are  a  step 
in  the  direction  of  more  efficient  results.  Such  ordinances  offer  a 
means  of  debarring  outfits  which  perform  unsatisfactory  work.  , 

EXTENT  AND  CHARACTER  OF  CITRUS  ORCHARDS. 

The  production  of  citrus  fruits  in  southern  California  is  confined  to 
the  narrow  stretch  of  land  south  and  w^est  of  the  Sierra  Madre  Range, 
extending  from  Santa  Barbara  on  the  north  to  the  Mexican  border. 
Although  citrus  plantings  are  located  here  and  there  throughout  this 
territory,  in  reality  only  a  small  proportion  of  the  land  capable  of 
cultivation  is  devoted  to  this  industry.  The  most  prominent  centers 
of  production  (see  fig.  1)  are  in  the  foothills  region  and  lower  land  of 
the  San  Gabriel  Valley;  the  corresponding  regions  of  the  San  Ber- 
nardino Valley,  including  the  Redlands-Highland,  Riverside,  and 
Corona  districts,  and  the  coast  region  of  Orange  and  Los  Angeles 
Counties.  Regions  of  smaller  production  are  found  in  southern 
Santa  Barbara  and  Ventura  Counties,  in  the  San  Fernando  Valley, 
and  in  western  San  Diego  County. 

The  groves  vary  in  size,  the  majority  probably  averaging  between 
5  and  15  acres.  Some  fruit  growers  have  from  50  to  100  acres  or 
more,  wThile  a  few  fruit  companies  control  from  several  hundred  up 
to  about  3,000  acres.  The  trees  for  the  most  part  are  budded  varie- 
ties which  average  less  than  20  feet  in  height.  In  some  districts  a 
few  groves  of  seedling  trees  30  to  35  feet  in  height  still  exist.  The 
trees  in  most  of  the  groves,  especially  those  of  more  recent  planting, 
are  regularly  arranged,  averaging  from  about  22  to  24  feet  apart. 
Some  of  the  older  groves  are  less  uniform,  either  because  they  were 
ttol  arranged  after  the  "block"  system,  or,  if  so,  additional  alternate 
rows  of  trees  were  interset,  which  broke  up  the  continuous  open  space 
between  two  rows  of  regularly  set  trees,  thus  rendering  it  confusing 
as  well  as  difficult  to  work  freely  therein. 


FUMIGATION  OF  CITRUS  TREES. 


7 


The  land  on  which  the  orchards  occur  is  for  the  most  part  flat  or 
only  gently  sloping,  and  in  a  state  of  frequent  tillage — conditions 
which  obtain  because  of  the  necessity  of  irrigating  during  much  of 
the  year.  At  Redlands,  in  San  Bernardino  County,  a  considerable 
acreage  of  oranges  is  found  on  terraced  land.  Fumigation  of  such 
trees  is  slow  and  difficult,  but,  fortunately,  they  comprise  a  very  small 
percentage  of  the  groves  in  that  county  requiring  treatment. 


INSECT  ENEMIES  OF  CITRUS  FRUITS.1 

The  larger  number  of  pests  most  injurious  to  citrus  fruits  in  south- 
ern California  belongs  to  the  Cocciche.  a  group  of  insects  popularly 


Fig.  1.— Map  showing  principal  localities  in  southern  California  where  citrus  fruits  are  produced. 

(Author's  illustration.) 


known  as  scale  insects.  Among  the  scale  insects  which  are  generally 
so  destructive  as  to  require  extended  efforts  for  their  control  are  the 
purple  scale  (Lepidosaphes  beckii  Newm.),  the  red  scale  (Ohrys&mr 
phalus  aurantii  Mask.),  and  the  black  scale  (Saissetia  olex  Bern.). 
The  yellow  scale  (Chrysom phalus  citrinus  Coq.),  considered  a  variety 
of  the  red  scale,  is  much  less  destructive  generally ,  t  Ik >ugh  sufficiently 
troublesome  in  some  localities  to  be  considered  a  pest  of  primary 
importance.  The  citrus  mealy  bug  (Pseudoeoccus  citri  Risso)  has 
recently  been  very  injurious  in  certain  sections.  Other  insect  pests 
attack  cit  rus  t  rees  to  a  greater  or  less  extent ,  but  those  just  mentioned 
are  generally  the  most  injurious,  and  the  principal  method  of  their 
control  is  fumigation  witli  hydrocyanic-acid  gas. 


i  See  Bui.  79,  Bur.  Ent.,  U.  S.  Dept.  Agr.,  1908,  p.  10. 
67330°— Bull.  90—12  2 


8 


HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


THE  PURPLE  SCALE. 

(Lepidosaphes  beckii  Newm.) 

The  purple  scale  appears  to  prefer  the  more  moist  regions  in  the 
vicinity  of  the  ocean,  as  its  distribution  is  confined  largely  to  this 
part  of  the  citrus  belt.  This  insect  confines  its  attacks  to  citrus  trees, 
infesting  not  only  the  leaves  and  branches  but  also  the  fruit.  Much 
injury  results.  The  young  purple  scale  insects  hatch  from  eggs 
deposited  by  the  adult.  The  number  of  broods  of  this  insect  in 
southern  California  has  never  been  exactly  determined.  Prof.  H.  J. 
Quayle,  of  the  University  of  California,  is  at  present  investigating  the 
life  history  of  this,  as  well  as  the  other  injurious  citrus  scale  pests.  In  a 
climate  like  that  of  southern  California,  which  is  never  severe  at  any 
time  of  the  year,  there  is  much  overlapping  of  broods,  so  that  scales  in 
all'stages  of  development  can  be  found  at  almost  any  time  of  the  year. 
The  writer's  own  observations  in  the  field  have  shown  that  there  are 
two  very  noticeable  general  broods,  one  appearing  in  the  early  spring 
during  March  or  April,  the  other  in  the  fall,  usually  about  October. 
These  broods  are  much  earlier  some  years  than  others,  depending  on 
the  nature  of  the  weather.  The  fall  brood  of  the  scale  is  the  most 
injurious,  as  shown  by  the  fact  that  trees  which  at  a  distance  may 
appear  entirely  healthy  one  month  may  have  the  leaves  of  a  large 
area,  or  even  an  entire  side,  turn  yellow  and  drop  off  the  next  month. 
The  orchardists  speak  of  this  as  "  firing. "  It  is  due  to  the  attacks  of 
the  enormous  number  of  young  insects  which,  on  hatching,  have 
spread  about  and  settled  down  on  those  branches  immediately 
adjoining  the  ones  previously  infested.  Trees  infested  with  the 
purple  scale  seldom  present  a  diseased  appearance  on  all  sides.  The 
habit  of  this  insect  is  to  frequent  the  inner  and  shadier  portions  of 
the  tree,  so  that  sometimes  severely  infested  trees  may  present  no 
visible  appearance  of  this  condition  on  the  outside.  In  the  majority 
of  cases  where  the  infestation  appears  on  the  outside  of  the  tree  it 
will  be  found  that  it  is  at  or  near  the  northwest  corner,  which  is  the 
shadiest  part  during  the  day.  The  attacks  are  also  confined  largely  to 
the  lower  part  rather  than  the  top  of  the  tree.  In  long  and  seriously 
infested  trees  the  insects  may  spread  throughout. 

THE  BLACK  SCALE. 

(Saissetia  olex  Bern.) 

The  black  scale  is  found  more  or  less  throughout  southern  Cali- 
fornia, yet  matures  more  freely  and  causes  more  injury  in  the  region 
adjacent  to  the  ocean  than  in  the  hot  interior  valleys.  It  occurs  on 
a  wide  range  of  hosts,  including  trees,  shrubs,  and  herbaceous  plants. 
The  commercial  importance  of  the  black  scale  arises  largely  from 


FUMIGATION  OF  CITRUS  TREES. 


9 


its  habit  of  secreting honeydew,  which  spreads  over  the  leaves,  fruit, 
and  brandies,  furnishing  a  growing  medium  for  a  black  or  sooty 
mold  fungus,  resulting  in  a  black  coating  throughout  the  tree.  This 
coating  is  removed  from  the  fruit  by  washing,  or  in  light  attacks  by 
brushing.  In  the  investigations  by  Dr.  (i.  Harold  Powell1  of  the 
causes  of  decay  of  oranges  while  in  transit  from  California,  it  was 
shown  that  the  decay  was  greater  in  washed  than  in  unwashed  fruit. 
To  avoid  the  washing  of  fruit  it  is  necessary  to  destroy  the  scale 
in  the  orchards.  The  black  scale  appears  generally  to  have  little 
effect  on  the  vitality  of  the  tree.  Its  attacks  are  confined  mainly 
to  the  brandies,  yet  it  is  commonly  found  on  the  leaves  during  its 
earlier  stages  of  development,  and  sometimes  it  matures  in  this  situa- 
tion. Seldom  doe>  it  mature  on  the  fruit.  The  young  of  the  black  scale 
insects  hatch  from  eggs  deposited  by  the  adult.     They  ran  be  readily 

destroyed  by  fumigation  in  the  early  stages  of  development.  Ap- 
proaching maturity  they  become  tough  and  leathery,  ami  in  this 
condition  they  are  capable  of  resisting  very  heavy  dosages  of  gas. 
The  breeding  of  the  black  scale  in  southern  California  has  never  been 
closely  investigated,  so  the  exact  number  of  broods  is  not  known. 
When  this  has  been  done  undoubtedly  it  will  be  found  to  be  very 
variable  with  different  hosts,  or  even  on  the  same  host.  The  scales 
on  "sucker"  shoots  will  mature  much  more  rapidly  than  those  on 
other  parts  of  the  tree.  There  is  one  noticeable  general  brood  which 
is  usually  hugely  hatched  by  the  first  part  of  September.  In  t  lie 
warmer  and  drier  parts  of  the  citrus  belt,  remote  from  the  coast,  the 

hatching  of  this  brood  is  quite  distinct,  so  that  in  most  instances  all 
the  insects  may  be  found  in  the  early  stages  of  development  at  the 
same  time.  In  the  immediate  vicinity  of  the  coast,  and  especially 
on  recently  budded  trees,  one  frequently  finds  the  scale  in  all  stages 
of  development  on  the  same  tree.  In  these  latter  instances  fumi- 
gation will  prove  less  satisfactory  than  in  the  former. 

THE  RED  SCALE. 

(Chrysom p/tulus  <iur<intii  If&ek.) 

The  red  scale,  although  its  injuries  are  more  severe  in  some  local- 
ities than  in  others,  has  the  limits  of  its  distribution  very  much  the 
same  as  has  the  black  scale.  It  can  be  found  within  a  few  miles  of 
the  ocean  or  as  far  inland  as  Iledlands.  This  insect  occurs  on  many 
host  plants  besides  citrus  trees.  It  attacks  the  fruit,  leaves,  and 
branches.  In  point  of  destructiveness  it  excels  all  other  citrus  scale 
insects  in  this  State,  destroying  not  only  branches,  but  sometimes 
entire  trees  by  its  attacks.  The  young  are  born  alive.  It  has  at 
least  three  broods  and  is  very  prolific. 


i  Bui.  123,  Bur.  Plant  Intl.,  U.  S.  Dept.  Agr.,  1908. 


10  HYDROCYAXTC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


THE  YELLOW  SCALE.  , 

(Chrysomphalus  citrinus  Coq.) 

Infestations  of  the  yellow  scale  appear  to  be  most  marked  in  the 
foothills  region  of  the  San  Gabriel  Valley  and  along  the  Sierra  Madre 
Range  through  Upland  and  Cueamonga.  It  causes  considerable  dam- 
age at  Redlands,  San  Bernardino  County,  yet  elsewhere  is  not 
regarded  as  a  very  serious  pest.  This  insect  infests  the  leaves  and 
fruit,  seldom  occurring  on  the  branches.  The  young  are  born  alive, 
as  in  the  case  of  the  red  scale,  to  which  it  is  closely  related. 

THE  MEALY  BUG. 

(Pseudoeocctts  citri  Risso.) 

The  mealy  bug  occurs  in  various  sections  of  the  southern  part  of 
California.  The  districts  of  greatest  injury  are  in  southern  San  Diego 
County  and  at  Santa  Paula,  in  Ventura  County.  Little  effort  for  its 
control  has  been  made  except  in  these  two  places.  This  insect  at- 
tacks fruit,  leaves,  and  branches,  secreting  a  honeydew,  which  is  fol- 
lowed by  a  black  fungus,  as  in  the  case  of  the  black  scale.  Its  injury 
is  much  greater  than  that  of  the  black  scale  because  it  discolors  and 
weakens  the  rind  of  the  fruit  at  those  places  where  it  extracts  the 
juice.  The  cottony  secretion  in  which  the  eggs  are  deposited  is 
difficult  to  remove.  The  severe  washing  which  this  fruit  requires, 
combined  with  its  weakened  rind  in  certain  places,  produces  a  heavy 
decay  in  such  fruits  as  are  treated  in  this  manner.  The  mealy  bug 
occurs  on  several  hosts  beside  citrus  trees.  The  young  are  hatched 
from  eggs  deposited  by  the  adult. 

APPARATUS. 

TENTS. 

When  hydrocyanic-acid  gas  was  first  employed  in  treating  orchards 
the  apparatus  used  in  the  process  was  of  a  very  cumbersome  nature.1 
The  most  popular  apparatus  consisted  of  tents  more  or  less  of  a  bell- 
shaped  nature  manipulated  by  a  high  derrick  mounted  on  a  wagon. 
The  wagon  was  drawn  between  the  rows  and  the  tents  lowered  over 
or  raised  from  the  trees  by  means  of  ropes  attached  to  the  derricks. 
The  use  of  such  apparatus' was  difficult,  slow,  and  costly. 

SHEET  TENTS. 

During  1892  Mr.  C.  W.  Finch,  a  fumigator  at  Riverside,  Cal., 
devised  8  much  simpler  and  cheaper  apparatus  than  those  theretofore 
used,  which  consisted  of  flat  sheet  tents,  octagonal  in  out  line.  (Fig.  7, 
p.  29.)     This  simplified  tent   was  rapidly  adopted,  and  now  sheet 


'  Report  of  Entomologist,  r.  s.  Dept.  Agr.,  1887,  p.  126. 


FUMIGATION  OF  CITRUS  TREES. 


1  1 


tents  are  exclusively  (parts  of  two  outfits  excepted)  used  in  southern 
California.  A  tent  of  this  character  is  easy  to  construct,  easy  to 
repair,  and  its  manipulation  has  been  so  perfected  by  years  of  use 
that  it  is  very  easily  handled  in  the  field  by  intelligent  workmen. 

Sizes. — The  standard  sizes  of  sheet  tents  an4  17,  24,  30,  36,  41,  43, 
45,  48,  52,  55,  and  64  feet,  but  larger  ones  up  to  72  and  S4  feet  have 
been  employed.  The  size  of  this  style  of  tent  is  properly  based  on 
the  distance  between  parallel  sides,  not  on  the  distance  between  oppo- 
site corners. 

Material*  used. — The  materials  now  generally  used  for  sheet  tents 
in  southern  California  are  6A-ounce  or  7-ounee  special  drills  and 
S-ounce  special  army  duck,  though  10-ounce  army  duck  is  sometimes 
used  in  very  large  tents.  These  cloths  are  spoken  of  in  ounces, 
meaning  such  a  weight  per  yard  :;n  inches  wide.  Drills  are  used  as 
Freely  as  ducks.  In  some  other  countries  where  fumigation  is  prac- 
ticed, notably  South  Africa,  even  heavier  than  10-ounce  cloth  is 
sometimes  used.  This  is  largely  because  of  its  strength  and  tightness 
of  texture.  The  tendency  in  California  has  been  to  sacrifice  tight- 
ness in  favor  of  lightness,  as  the  lighter  tents  are  so  much  more  easily 
manipulated  in  the  field.  The  main  reason  for  this  tendency  is  prob- 
ably that  the  practice  has  been  largely  in  the  hands  of  contract  fumi- 
gators  rather  than  in  the  hands  of  the  growers  themselves.  The  con- 
cerns furnishing  the  fumigation  t cuts  apparently  have  made  no  special 
effort  to  supply  the  very  tightest  goods  available  on  the  market,  prob- 
ably because  the  profits  on  these  goods  would  be  smaller  than  on 
the  cheaper  and  more  porous  cloths.  Several  of  these  firms  have 
special  goods  which  they  recommend  for  fumigation  use.  For  the 
most  part  these  goods  are  about  on  a  par  as  regards  the  requirements 
for  fumigation.  Only  one  grade  distinctly  superior  to  the  others  has 
been  seen;  it  is  used  solely  by  private  outfits,  and  is  slightly  more 
expensive  than  the  other  grades.  The  results  secured  depend  di- 
rectly on  the  tightness  of  the  cloth:  in  fact,  this  consideration  of 
tightness  oi  tenting  is  one  of  the  most  important  factors  in  the  entire 
fumigation  procedure.  On  it  depends  not  only  the  efficacy  of  the 
treatment  but  also  to  some  extent  the  cost  of  the  operation.  A 
dosage  recommended  as  securing  certain  results  with  tents  of  a  given 
degree  of  tightness  will  not  produce  the  same  results  with  tents  of 
less  closely  woven  material.  Even  though  the  initial  cost  is  greater, 
tightly  woven  material  is  the  most  economical  in  the  long  run. 

Ni  w  tenting  material. — Considerable  attention  has  been  given  dur- 
ing this  investigation  to  the  character  of  cloth  used  in  fumigating 
tents,  and  an  attempt  has  been  made  to  secure  the  most  suitable 
material  possible.  The  leading  manufacturers  and  dealers  in  cotton 
ducks  and  drills  in  the  United  States  were  consulted  and  samples  of 
their  tightest  cloths  secured.    Many  of  the  nearly  two  hundred  sam- 


12  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


pies  secured  were  very  superior  in  tightness  to  the  grades  now  gen- 
erally supplied  in  California  for  the  fumigation  trade.  Although  this 
collection  contained  samples  of  the  very  tightest  ducks  and  drills 
manufactured,  it  was  decided  to  accept  the  offer  of  one  of  the  largest 
cotton-goods  concerns  to  carry  on  special  experiments  in  weaving  for 
the  benefit  of  this  investigation.  As  a  result  of  these  experiments 
samples  of  6-ounce,  7-ounce,  and  8-ounce  drills  were  furnished. 
These  are  easily  the  most  tightly  woven  drills  the  writer  has  ever 
seen  and  some  local  cloth  experts  are  of  the  same  opinion.  Each  of 
these  samples  was  made  very  tight  by  forcing  in  more  threads  than 
are  found  in  the  tightest  drills  on  the  market. 

Experiments  with  new  tent  material. — Tents  were  made  of  each  of 
these  7-ounce  and  8-ounce  special  drills  and  tested  in  the  field.  Part 
of  an  orange  orchard  of  trees  from  10  to  15  feet  tall  which  were 
severely  infested  with  the  purple  scale  was  fumigated  during  Sep- 
tember, 1909.  The  point  was  to  determine  at  what  strength  eradi- 
cation would  occur  with  these  new  tents.  Both  potassium  cyanid 
and  sodium  cyanid  were  used.  On  examining  the  results  with  these 
new  tents  it  was  found  that  schedule  No.  1  (see  p.  34)  with  potas- 
sium cyanid  produced  eradication,  whereas  with  the  ordinary  fumiga- 
tion tents  a  14  schedule  was  required  to  secure  the  same  results. 
With  the  sodium  cyanid  it  was  found  that  the  equivalency  of  between 
a  three-fourths  schedule  and  a  No.  1  schedule  produced  eradication, 
whereas  it  requires  the  equivalent  of  a  1J  schedule  with  ordinary 
tents  to  reach  the  same  degree  of  efficiency.  These  results  with  both 
potassium  cyanic!  and  sodium  cyanid  show  that  this  new  tenting 
material  requires  at  least  one-fourth  less  of  these  chemicals  than  the 
regular  tents  now  largely  tised.  This  would  mean  a  saving  of  fully 
25  per  cent  on  the  amount  of  cyanid  required  in  field  work  if  the 
tighter  tents  arc  used. 

What  cloth  to  use. — Cyanid  is  the  most  expensive  element  in  fumi- 
gation work.  A  saving  of  25  per  cent  on  this  article  means  a  materi- 
ally lessened  cost  for  the  process.  This  better  and  tighter  special 
tenting  material  may  be  somewhat  more  expensive  than  the  present 
inferior  goods  used,  yet  the  amount  of  cyanid  saved  as  well  as  the 
superior  results  secured  from  its  use  will  in  the  long  run  many  times 
offset  the  additional  initial  cost.  The  writer  would  advise  either  a 
7-ounce  or  an  N-ounce  weight  of  these  new  goods  for  commercial 
fumigation  as  superior  to  any  cloth  he  has  ever  seen.  There  appears 
to  be  little  difference  in  tightness  between  the  two  weights.  By 
reason  of  its  greater  weight  the  S-ounce  drill  might  prove  more -dura- 
ble, whereas  on  the  other  hand  the  7-ounce  weight  is  easier  of  manip- 
ulation in  the  Held.  This  special  grade  of  drill  advised  by  this  inves- 
tigation can  be  purchased  at  any  of  the  dealers  in  fumigation  tents, 
in  Los  Angeles,  Cal.    Anyone  making  an  investment  of  the  amount 


FUMIGATION  OF  CITRUS  TREKS. 


13 


necessary  in  the  purchasing  of  a  fumigation  outfit  usually  is  better 
satisfied  if  he  has  exercised,  or  had  the  opportunity  to  exercise,  his 
own  judgment  in  the  selection  of  the  cloth,  even  if  experience  has 
proved  one  particular  kind  to  be  superior  to  all  others.  To  satisfy 
this  demand  of  independent  people  it  is  suggested  that  anyone  con- 
templating the  purchase  of  a  fumigation  outfit  procure  as  many 
samples  as  possible  of  6^-ounce  to  8-ounce  drills  and  S-ounce  ducks 
and  compare  the  tightness  of  these  samples  with  that  of  samples  of 
the  7-ounce  and  8-ounce  special  drills  recommended  by  this  investi- 
gation. This  can  he  done  by  holding  the  various  samples  at  dill'erent 
angles  between  the  eye  and  the  sun,  noting  the  comparative  number 
of  light  rays  penetrating  the  different  sample-. 

Construction . —  In  California  the  demand  for  fumigation  tents  is  so 
great  that  several  concerns  make  a  special  business  of  meeting  it. 
The  man  contemplating  the  purchase  of  an  outfit  visits  one  or  all  of 
these  different  dealers,  and,  having  selected  the  cloth  which  meets 
with  his  approval,  places  his  order  for  the  number  of  tents  of  the 
size  desired.  These  are  shipped  to  him  ready  for  use.  A  method  of 
constructing  tents  will  bp  explained,  however,  for  the  benefit  of 
people  more  distant  from  the  sources  of  supply  than  are  the  people 
of  southern  California.  As  previously  explained,  the  tents  are  flat 
sheets,  octagonal  in  shape.  The  duck-  and  drills  are  usually  30 
inches  wide,  although  sometimes  they  measure  L,(.>  or  29£  inches. 
The  sides  of  the  strips  are  sewed  together  so  that  the  strips  all  run 
in  a  parallel  direction.  Before  attempting  t<>  cut  the  cloth  for  a 
tent  it  is  well  to  construct  a  diagram  having  therein  an  exact  rep- 
resentation of  the  number  of  strips  required  as  well  as  their  length  and 
shape.  Such  a  diagram  is  shown  in  figure  2  (p.  l  i  ).  and  was  originally 
presented  by  Dr.  A.  \Y.  Morrill,1  who  was  engaged  in  fumigation 
Work  against,  citrus  pests  in  Florida  from  lu07  to  190!).  Each  side 
of  the  tent  or  oct  agon,  when  const  ructed,  will  be  equal,  approximately, 
to  two-fifths  of  the  distance  between  the  parallel  sides.  In  explaining 
the  construction  Dr.  Morrill  states : 

The  si  rips  when  cut  should  bo  overlapped  three-eighths  or  one-half  inch  and  double 
stitched,  and  all  raw  edges  should  be  hemmed.  In  calculating  the  number  and 
length  of  strips  the  overlapping  will  reduce  the  width  of  the  cloth  from  three-fourths 
inch  to  1  inch.  As  an  illustration  of  the  method  of  calculating  the  length  of  the 
strips  used  in  making  an  octagonal  tent  of  8-ounce  duck,  50  feet  may  be  taken  as  the 
desired  size.  This  is  equal  to  GOO  inches  and  the  width  of  the  cloth,  if  29.5  inches, 
will  be  reduced  to  28.5  if  overlapped  one-half  inch  at  the  seams.  By  dividing  28.5 
inches  into  inches  the  nearest  multiple  is  found  to  be  598.5  inches,  or  49  feet 
10^  inches,  which  is  sufficiently  close  to  the  desired  width  for  practical  purposes. 
The  number  of  strips  in  a  tent  598.5  inches  wide  is  21.  The  middle  section  B  [fig.  2] 
is  approximately  ,two-fifths  the  entire  width,  or  239.5  inches.  Deducting  this  from 
598.5  inches,  the  entire  width,  the  remainder,  359,  equals  the  sum  of  the  widths  of 
sections  A  and  C.    These  sections  being  equal,  the  width  of  each  is  179.5  inches. 


i  Bui.  7«,  Bur.  Ent.,  U.  S.  Dept.  Agr.,  p.  10,  1908. 


14       Hydrocyanic-acid  gas  fumigation  in  California. 


The  number  of  strips  in  each  section  can  now  be  readily  calculated.  The  21  strips 
should  be  numbered  on  the  diagram  from  left  to  right.  .  Section  A  requires  six  strips 
and  8.5  inches  of  the  seventh.  Similarly,  section  C  requires  six  strips  beginning  at 
the  right  (twenty-first  to  sixteenth,  inclusive)  and  8.5  inches  of  the  fifteenth.  Sec- 
tion B  requires  the  remaining  20  inches  of  strip  No.  7,  20  inches  of  strip  No.  15,  and 
seven  entire  widths,  thus  making  the  total  of  21  strips  required. 

The  cutting  of  the  cloth  can  be  done  without  waste  if  the  details  of  construction 
are  well  planned.  In  the  above  tent  seven  strips  50  feet  long  (49  feet  10£  -inches) 
should  first  be  cut  out  for  section  B.  Strips  Nos.  7  and  15  are  next  cut  and  the  out- 
side corners  cut  at  an  angle  of  45  degrees,  as  indicated  in  the  diagram.  Each  strip 
for  sections  A  and  C  is  cut  shorter  by  its  own  width  outside  at  each  end  than  the  strip 

preceding  it.  Thus  the 
required  lengths  of  the 
side  strips  are  found  by 
matching  the  inner  edge 
of  the  new  one  to  the 
outer  edge  of  the  one  be- 
fore it.  It  is  desirable  to 
have  the  central  section, 
B,  made  up  entirely  of 
full-length  strips  so  that 
the  stress  will  not  be 
across  seams.  The  stress 
is  so  slight,  compara- 
tively, in  the  side  sec- 
tions A  and  C  that  this  is 
not  an  important  point. 

Such  is  the  con- 
struction of  a  50-foot 
tent.  The  method  of 
constructing  a  tent 
of  any  other  size  is 
similar.  Tents  up  to 
45-foot  size  are  con- 
structed throughout 
of  either  of  the  drills 
or  of  8-ounce  duck. 
Larger  size  tents  should  have  the  full  length  strips  of  8-ounce  duck, 
while  the  shorter  side  strips,  or  "skirts,"  as  they  are  sometimes 
called,  are  made  of  a  light  drill.  The  main  strain  and  wear  falls  on 
the  middle,  heavier  and  stouter,  long  strips;  hence  the  use  of  the 
lighter  material  for  the  "skirts"  decreases  the  weight  of  the  tent 
without  affect  ing  its  durability.  The  duck  used  in  such  tents  should 
be  of  the  very  tightest  grade  available,  while  the  6£-ounce  or  7-ounce 
special  drill  previously  recommended  is  most  suitable  for  the  "skir  s." 

Amount  of  cloth  required  for  different-sized  tents. — It  is  very  essential 
in  constructing  tents  to  know  the  amount  of  cloth  required  for  the 
size  which  it  is  intended  to  make.  The  writer  has  calculated  this  for 
the  regular  sizes  and  gives  below  the  results.    Calculations  are  based 


E 


ct  I  b-  -£  f  c 


Fig.  2.— Plan  for  construction  of  octagonal  sheet  tent  50  feet  across, 
showing  lines  used  in  constructing  octagon:  A,  C,  side  sections;  B, 
central  section  of  full-length  strips;  E,  E,  so-called  "ends"  of  tent; 
S,  S,  so-called  "sides"  of  tent;  R,  R,  reinforcements;  1-21,  strips 
of  duck  29 J  inches  wide  overlapped  £  an  inch  at  the  seams.  (From 
Morrill.) 


FUMIGATION  OF  CITRUS  TREES. 


15 


on  cloth  30  inches  wide,  and  represent  the  number  of  linear  yards 
of  such  cloth  required — not  square  yards.  Allowance  of  an  inch  to 
each  strip  has  been  made  for  overlapping  edges.  These  figures  are 
based  on  the  assumption  that  the  cloth  is  cut  without  waste. 


Size  of  tent. 

Cloth 
required. 
(30  inches 

wide.) 

Size  of  tent. 

Cloth 
required. 
(,:*)  inches 

wide.) 

24  feet  

Yards. 

70 
105 
155  I 
195 
215 

m 

Yards. 
2u5 
315 
345 
470 
590 

30  feet  

52  feet  

36  feet  

55  feet  

41  feet  

64  feet  

43  feet  

72fMt  

45  feet  

Size  to  purchase .— Most  <>f  the  tents  in  southern  California  are 
of  either  the  36,  41,  43,  or  45  foot  sizes.  Few  tents  of  less  than  36- 
foot  size  are  constructed.  Out  fit  ~-  or  parts  of  outfits  having  tent^  of 
48,  50,  52,  and  various  other  sizes  as  high  as  84-fbot  are  known.  The 
number,  however,  is  comparatively  small.  The  size  of  tent  required 
depends  on  the  size  of  trees  in  the  orchard  or  orchards  to  be  fumi- 
gated. The  tents  should  be  large  enough  to  cover  the  taDesi  trees. 
An  easy  method  of  accomplishing  this,  as  suggested  by  Morrill,  is  by 
th  rowing  a  tape  attached  to  a  reel  over  the  top  of  the  tallest  tree  and 
measuring  from  ground  to  ground.  Although  the  weight  of  the  tent 
reduces  the  height  of  the  tree  to  some  extent,  nevertheless  it  is  policy 
to  add  from  2  to  4  feet  to  the  distance  measured  by  the  tape  so  as  to 
be  assured  of  having  the  edges  of  the  tent  rest  well  on  the  ground. 
If  an  outfit  is  to  be  procured  for  use  in  a  young  orchard,  the  tents 
purchased  should  be  large  enough  to  allow  for  5  or  6  years  extra 
growth.  The  average  age  of  a  fumigation  outfit  is  from  3  to  5  years, 
depending,  of  course,  on  the  amount  and  character  of  usage  which  it 
has  undergone.  A  well-cared-for  outfit  used  only  by  a  private 
grower  in  covering  his  own  orchard  should  last  through  5  or  (i  seasons 
of  work. 

Ring  attachments  and  re  en  for  cements. — Small  iron  rings  are  some- 
times attached  to  tents  as  catch  places  for  the  poles  or  derrick  hooks 
used  in  throwing  them  over  trees.  These  ring  attachments  are  most 
convenient  on  tents  above  45  feet  in  diameter,  but  unnecessary  on 
smaller  sizes.  An  easy  and  satisfactory  method  of  attaching  rings 
to  the  cloth, as  proved  by  many  years  of  experience  in  California  and 
elsewhere,  is  shown  in  figure  3.  It  consists  in  gathering  the  cloth 
of  the  tent  about  some  object  or  material,  binding  the  same  in  place 
by  a  stout  cord,  which  also  passes  through  the  ring.  .V  tightly  rolled 
wad  of  some  cloth  such  as  burlap  is  commonly  used.  Another  method 
well  worth  mentioning  is  by  means  of  a  piece  of  manila  rope  from 


16  HYDKOCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

3  to  4  feet  long  sewed  to  the  tent  in  the  form  of  a  right  angle,  as  shown 
in  figure  7,  page  29.  As  fumigation  tents  should  be  pulled  onto  or  off 
the  trees  in  the  direction  in  which  the  strips  of  cloth  run,  the  rings 
should  be  so  placed  as  to  make  this  method  of  manipulation 
possible.  This  is  accomplished  by  having  two  rings  at  either  end  of 
the  tent  and  apart  by  about  the  width  of  the  average-size  tree  treated. 
They  should  be  placed  from  3  to  5  feet  back  from  the  edge,  the  dis- 
tance depending  on  the  size  of  the  tent.  A  small  link  of  chain 
called  a  "jingler"  is  usually  attached  to  the  ring,  the  sole  purpose 
of  which  is  to  direct  the  operator  to  its  location.  By  merely  giving 
the  sheet  a  shake  this  simple  device  enables  the  tent  pullers  to 

easily  locate  the  rings  on  the  darkest  nights. 
Such  a  great  strain  is  localized  at  the  place 
where  the  rings  are  attached  that  it  is  well 
to  have  this  part  reenforced  by  stitching 
on  an  extra  thickness  of  cloth.  The  same 
material  of  which  the  tent  is  constructed 
is  very  suitable.  The  strip  used  should  be  3 
or  4  feet  long. 

BELL  TENTS. 

Originally  the  bell,  or  hoop,  tent  was  the 
kind  in  use  in  California,  and  even  now  it  en- 
joys a  limited  use  in  some  countries.  This 
tent  is  dome  shaped,  having  the  mouth  held 
open  by  a  circle  of  f-inch  gas  pipe.  It  is 
suited  only  for  covering  small  trees.  Plate  I 
illustrates  the  character  of  this  tent  and  the 
method  of  its  manipulation.  Experiments  in 
California  have  resulted  in  the  disuse  of  bell 
tents  in  favor  of  sheet  tents,  the  latter  style 
being  not  only  easier  of  construction  and  manipulation,  but  also 
more  easily  kept  in  repair, 

GAS-PROOFING. 

The  treatment  of  covers  with  various  substances  to  increase  their 
tightness  has  been  in  practice  to  a  greater  or  less  extent  since  the 
beginning  of  fumigation.  Linseed  oil  was  one  of  the  first  tried. 
It  renders  the  tents  perfectly  tight  but  greatly  increases  their  weight. 
Experience  has  proved  that  tents  so  treated  are  liable  to  burning  and 
rotting  under  the  conditions  to  which  they  are  subjected  in  the  field. 
Treating  tents  with  the  mucilaginous  concoction  resulting  from 
soaking  the  common  cactus  (Opuntia  engelmanni)  in  water  for 
two  to  four  days  was  practiced  to  some  extent'  during  the  nineties. 
Numerous  other  methods  have  been  tried,  such  as  painting  with  a 
flexible  paint;  treating  with  glue  dissolved  in  water;  treating  with 


Fig.  3.— Method  of  attaching 
hooks  to  tent  when  covernig 
trees  with  aid  of  derricks:  a, 
Tent  gathered  around  ball  of 
burlap  or  other  suitable  ob- 
ject; b,  stout  cord  for  attach- 
ing ring;  c,  catch  ring;  d,  hook 
on  pulley  block;  e,  lap  link 
or  "jingler."    (From  Morrill.) 


Bui.  90,  Part  I,  Bureau  of  Entomology,  U.  S.  Dept.  of  Agriculture 


Plate  I. 


Figs.  1-3. -Method  of  Covering  Small  Tree  with  Bell  or  Hoop  Tent.  (From 

Morrill.  I 


FUMIGATION  OF  CITRUS  TREES. 


17 


•actus  juice  combined  with  linseed  oil,  or  glue,  or  tannin,  etc.  Expe- 
rience with  these  many  substances  has  resulted  in  discarding  all  of 
them  in  this  State,  so  that  to-day  no  effort  is  made  to  render  tents 
gas-tight  by  the  use  of  any  liquid  substance. 

It  is  well  known  that  a  decidedly  smaller  dosage  would  be  required 
with  cas-ti<rht  covers  than  is  nesessarv  with  tents  of  the  present 
character.  This  has  Led  many  writers  on  fumigation  to  advise  the 
treatment  of  the  cloth  with  some  of  the  substances  just  mentioned  to 
accomplish  that  end.  Had  these  writers  undergone  a  considerable 
experience  in  the  actual  handling  of  tents  in  the  field  their  expressions 
on  this  particular  subject  might  have  been  somewhat  modified. 
Tents  treated  by  some  of  t hoe  methods  will  be  rendered  gas-tight ,  or 
Dearly  BO,  but  for  certain  practical  reasons  they  are  not  now  used  and 
never  will  be  used  on  a  large  commercial  scale. 

Experiments  have  been  made  with  many  different  substances  in 
attempting  to  render  cloth  gas-tight,  and  several  samples  of  gas-tight 
or  almost  gas-tight  cloth  have  been  received  from  dealers.  This 
experience  ill  all  cases  has  shown  that  to  render  a  cloth  very  much 
more  nearly  gas-tight  than  is  possible  in  weaving,  some  treatment 
must  be  used  which  materially  increase's  the  weight  of  the  tent  as 
well  as  rendering  it  somewhat  still*.  Both  of  these  conditions  should 
be  avoided  as  much  as  possible.  Heavy  tents  are  not  only  difficult 
to  manipulate,  but  also  destroy  fruit  and  break  blanches  while  being 
hauled  over  trees.  Stiff  tents  will  not  lie  close  to  the  ground,  thus 
allowing  the  escape  of  gas.  Tents  must  be  constantly  overhauled  to 
mend  the  holes  which  result  from  acid  burns  as  well  as  other  causes. 
The  mending  of  such  t  reated  cloth  is  so  dillicult  as  to  be  impracticable, 
especially  in  large-sized  tents. 

After  considering  both  sides  of  the  question  experience  leads  to 
the  conclusion  that  the  economy  of  gas  resulting  from  gas-tight  tents 
is  more  than  offset  by  the  many  difficulties  experienced  in  the  use  of 
such  stiff  heavy  covers  in  the  held.  The  writer  advises  the  purchase 
of  the  most  closely  woven  untreated  cloth  obtainable,  of  the  char- 
acter and  weight  previously  mentioned  (p.  12),  believing  such  to  be 
superior  for  orchard  work  to  cloths  which  have  undergone  a  treatment 
to  render  them  gas-tight. 

Small  tents  used  in  treating  nursery  stock,  and  especially  covers  of 
cloth  made  over  a  framework  in  the  shape  of  a  box  having  one  end 
open  so  as  to  be  easily  placed  over  nursery  trees  or  such  small  plants, 
can  be  rendered  gas-tight  without  experiencing  some  of  the  more 
serious  objections  to  their  practical  use  that  exist  in  the  case  of  large 
covers  in  orchard  work.  Linseed  oil  is  very  suitable  for  this  purpose. 
The  preparation  and  application  of  a  linseed-oil  varnish,  which  is 
used  by  the  War  Department  in  the  treatment  of  cloth  for  balloon 
purposes,  is  quoted  below.    It  renders  the  cloth  gas-tight  and  at  the 


18  HYDROCYANIC- ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


same  time  leaves  it  more  pliable  than  any  other  gas-tight  treatment 
experienced.  The  Acting  Secretary  of  War  has  described  this 
method  as  follows: 

In  order  to  render  the  cioth  gas-tight  a  linseed-oil  varnish  is  applied.  The  varnish 
is  made  in  proportion  of  100  pounds  pure  linseed  oil,  4  pounds  litharge,  and  1  pound 
of  umber.  This  should  be  heated  to  a  temperature  of  130°  to  200°  C.  for  six  or  seven 
hours  and  constantly  stirred  during  that  time.  A  sponge  or  wad  of  cloth  is  ordinarily- 
used  for  applying  the  varnish,  which  should  be  put  on  in  very  thin  coats  and  well 
rubbed  in  by  hand.  The  addition  of  one  coat  of  this  varnish  about  once  a  year  will 
be  found  of  great  value  in  preserving  the  impermeability  of  the  material. 

One  coat  of  this  oil  on  each  side  of  the  tenting  will  prove  adequate 
with  most  cloths.  Treated  cloth  should  be  hung  up  to  dry  for 
about  two  weeks,  and  if  not  entirely  tight  at  the  end  of  that  time  a 
second  coat  should  be  applied.  The  cloth  should  be  thoroughly  dry 
before  it  is  used. 

MILDEW-PROOFING. 

The  treatment  of  tents  with  substances  to  render  them  proof 
against  mildew  is  practiced  to  some  extent.  In  San  Bernardino  and 
Riverside  Counties  probably  the  majority  of  tents  are  dipped  in  a 
solution  of  tannin,  while  in  Los  Angeles  and  Orange  Counties,  which 
are  much  nearer  the  coast  and  consequently  have  a  more  generally 
moist  climate,  tents  for  the  most  part  are  used  without  any  treatment 
whatever.  This  localizing  of  treatment  to  the  dryer  sections  demon- 
strates that  for  a  climate  like  that  of  southern  California  the  mildew- 
proofing  of  tents  is  not  absolutely  essential.  The  covers  used  in  this 
investigation  have  been  in  use  mostly  in  the  more  moist  coastal 
region  for  three  seasons,  yet  they  have  never  been  affected  with 
mildew.  Neither  has  any  mildew  injury  to  other  tents  been  seen  or 
heard  of  meanwhile.  The  life  of  untreated  tents  in  this  State  appears 
to  be  as  long  as  that  of  those  which  have  been  mildew-proofed — at 
least  this  is  the  case  with  tents  that  are  properly  cared  for  in  the 
field  as  well  as  in  storage. 

Long-used  tents  are  now  cast  aside,  not  because  of  weakness  due 
to  deterioration  of  cloth  from  mildew,  but  largely  from  weakness  due 
to  extensive  mending  of  holes,  resulting  principally  from  acid  burns, 
but  to  some  extent  also  from  use  on  trees  containing  dead  branches. 
This  necessary  patching,  combined  with  general  wear,  limits  the  life 
of  the  average  fumigating  tent  to  3  or  4  years.  Judging  from,  the 
experience  of  California  fumigators,  as  well  as  from  that  of  the  writer 
himself,  it  appears  unnecessary  in  California  to  treat  tents  for  mildew 
if  proper  precautions  are  taken  for  drying  them.  Wet  tents  should 
be  spread  out  during  the  day  on  the  ground  between  the  trees,  so 
I  bat  the  sun  may  reach  them  as  much  as  possible.  At  the  end  of  a 
season's  work  they  should  be  thoroughly  dried,  rolled  up,  and  stored 
in  a  dry  room. 


FUMIGATION  OF  CITRUS  TREES. 


19 


In  places  like  Florida,  as  well  as  in  tropical  countries  where  tents 
become  wet  every  night,  treatment  to  prevent  mildew  would  seem 
advisable.  Even  in  California  it  will  act  as  a  guarantee  to  those 
fumigators  who  exercise  little  care  in  the  drying  of  their  outiit. 
The  dipping  and  boiling  of  tents  in  a  solution  of  tannin  is  the  only 
method  now  practiced  there  to  render  them  proof  against  mildew. 
This  tannin  treatment  has  been  in  use  for  a  long  time,  and  is  very 
satisfactory.  Contrary  to  the  belief  of  many,  tannin  does  not  render 
the  tents  any  tighter.  It  merely  shrinks  them,  which  can  be  as  well 
accomplished  by  dipping  in  water,  or  a  few  nights'  exposure  to  heal  y 
dews  will  produce  the  same  results.  The  tannin  treatment,  as  prac- 
ticed by  Mr.  S.  A.  Pease,  horticultural  commissioner  of  San  Bernar- 
dino County,  is  a>  follow-: 

A  brick  furnace  [Plate  II,  tig.  1|  is  constructed  mi  that  the  upper  half  partially 
incloses  a  tank.  :i  by  10  feet  and  :i  feet  deep,  made  of  \<».  ](>  galvanized  iron.  This  is 
filled  with  water  to  within  8  inches  of  the  top,  which  would  be  about  500  gallons,  and 
about  200  pounds  of  extract  of  oak  bark  is  added.  This  mixture  is  raised  to  a  tem- 
perature as  high  as  the  hands  of  the  operators  will  .-land.  A  tent  stretched  out  in  as 
loosened  a  condition  as  possible  ( not  in  a  wad  i  is  then  introduced  into  the  vat .  It  is 
stirred  around  and  kept  submerged  by  means  of  wooden  paddles  manipulated  by  the 
crew.  After  20  or  30  minutes  of  this  treatment  the  tent  is  raised  to  the  top  of  a  derrick 
above  the  tank  and  suspended  for  a  few  minutes  until  well  drained,  after  which  it  is 
lowered  on  a  rack,  moved  away,  and  spread  out  «>n  the  ground  to  dry.  Twenty-five 
gallons  of  water  and  20  pounds  of  extract  are  now  added  to  the  tank  before  another 
tent  is  introduced,  and  this  is  repeated  for  each  succeeding  one. 

The  above  recommendations  are  for  45-foot  tents.  Larger  tents 
require  more  material  and  smaller  ones  less.  Oak-bark  extract  costs 
about  10  cents  per  pound  by  the  barrel. 

MAliKIM,. 

On  pages  29-30  of  this  bulletin  is  explained  a  method  of  marking 
tents,  which  is  used  in  the  most  improved  fumigation  procedure. 
Tents  should  have  been  thoroughly  wet  at  Least  once  before  being 
marked,  as  new  cloth  is  subject  to  considerable  shrinkage,  and  if 
marked  before  shrinkage  the  measurements  will  be  erroneous.  Meas- 
urements made  of  several  tents  of  6\-ounce  drill,  before  and  after 
shrinkage,  showed  that  45-foot  covers  shrink  about  3  feet  length- 
wise of  the  strips  of  cloth.  The  crosswise  shrinkage  is  much  less.  A 
convenient  method  of  shrinking  untreated  tents  is  to  spread  them 
out  on  the  lawn  and  wet  with  a  hose  or  sprinkler.  After  being  dried 
they  are  ready  for  marking.  Tents  treated  with  tannin  should  be 
marked  after  the  treatment. 

The  best  method  for  marking  tents  is  to  place  them  on  a  smooth 
floor.  If  this  is  not  possible,  spread  them  out  on  the  smoothest 
ground  available.  A  tapeline,  brush,  and  marking  fluid  are  required. 
Printer's  ink,  diluted,  is  the  best  marking  material,  although  some 


20  HYDROCYANIC- ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


have  used  lampblack  and  turpentine,  or  a  soft,  flexible  paint,  with 
satisfaction.  The  first  line  marked  should  be  the  one  running 
through  the  center  of  the  tent.  When  many  tents  are  to  be  prepared, 
the  use  of  a  stencil  large  enough  to  include  a  complete  line  of  figures 
on  one  side  will  facilitate  the  operation.  The  numerals  should  be 
not  less  than  5  or  6  inches  long. 

POLES  AND  DERRICKS. 


Two  wooden  poles  or  derricks  are  used  in  placing  tents  over  trees. 
No  absolute  statement  can  be  made  as  to  when  poles  should  be 
employed  or  when  derricks.    The  practice  is  to  use  poles  when- 

ever  possible. 
This  has  resulted, 
in  general,  in  the 
use  of  poles  with 
tents  up  to  45  feet 
in  size  and  of 
derricks  with  the 
larger  sizes.  Some- 
times poles  are 
used  with  tents  of 
48-foot  or  50-foot 
sizes,  but  this  is 
difficult,  especially 
if  the  trees  are  tall 
and  narrow  rather 
than  low  and 
broad.  Preferably 
the  poles  should 
be  6  inches  to  a 
foot  longer  than 
the  height  of  the 
trees.  The  two 
lengths  of  poles  in  most  common  use  are  14  feet  and  16  feet.  Twenty- 
foot  poles  are  occasionally  required.  These  poles  average  from  2  inches 
to  2\  inches  in  diameter,  are  rounded,  and  made  of  straight-grained 
Oregon  pine.  The  lower  end  is  slightly  sharpened  to  secure  a  ready 
hold  in  the  ground.  The  upper  end,  to  which  a  rope  is  attached  for 
erecting  the  poles,  preferably  is  also  bluntly  narrowed  after  one  of 
the  methods  shown  in  figure  4.  This  figure  also  shows  two  convenient 
methods  of  attaching  the  rope.  The  end  of  a  is  narrowed  about  one- 
half  inch  on  all  sides  for  3  or  4  inches.  This  allows  it  to  slip  easily 
through  the  rings  in  tents.  The  rope  is  tied  in  a  shallow  furrow  6  or 
7  inches  distant  from  the  end.  In  h  the  end  of  the  pole  is  merely 
rounded,  w  hile  the  rope  occupies  an  auger  hole  through  the  center 


Fig.  4.— Ends  of  hoisting  poles  used  in  placing  tents  over  trees:  a,  Used 
with  tents  where  rings  are  present:  fe,  used  with  tents  having  no  rings. 
(Original.) 


Bui.  90,  Part  I,  Bureau  of  Entomology,  U.  S.  Dept.  of  Agriculture. 


Plate  II. 


Fig.  1.— Brick  Furnace,  Tank,  and  Derrick  Used  in  the 
Tannin  Treatment  of  Tents  to  Prevent  Mildew, 
San  Bernardino  County,  Cal.  'Original.' 


Fig.  2.— Machine  for  Covering  Tree  with  Sheet  Tents.   Devised  3y  C.  E. 
McFadden,  of  Fullerton,  Cal.  'Original.' 


FUMIGATION  OF  CITRUS  TREES. 


21 


of  the  pole  and  at  the  same  distance  from  the  end  as  is  the  furrow 
in  a.  A  knot  at  the  end  of  the  rope  prevents  its  removal  from  the 
hole. 

Most  of  the  wear  on  the  rope  falls  on  the  first  2  or  3  feet  adjacent 
to  the  pole,  for  this  part  is  hall-hitched  each  time  over  the  tent. 
(See  PI.  Ill,  fig.  1.)  Some  fumigators  substitute  a  stout  piece  of 
rawhide  about  3  feet  long  to  occupy  this  region  between  the  end  of 
the  rope  and  the  pole.  The  rope  should  be  J-inch  or  f-inch,  and 
about  3  feet  longer  than  the  pole. 

Derricks  are  necessary  for  very  tall  trees,  which  can  not  be  covered 
conveniently  with  poles.  They  consist  of  long  poles  having  a  frame- 
work attached  to  the  bottom  by  which  to  prevent  slipping,  as  well 
as  to  confine  their  movement  to  either  of  two  directions  while  stand- 
ing erect.  There  is  a  rope  and  pulley  arrangement  at  the  top  for 
raising  the  tent.  The  length  of  the  uprights  depends  on  the  height 
of  the  trees  to  be  fumigated.  They  should  be  fully  a  foot  higher  than 
the  tallest  trees.  \\\  California  derricks  average  between  1>">  and  35 
feet  tall,  having  the  top  2\  to  Z\  inches  and  the  bottom  3\  to  4J 
inches  in  diameter.  Straight-grained  Oregon  pine  is  used.  Then 
construction  is  well  shown  in  Plate  111,  figures  2  and  .*'>.  The  frame- 
work at  the  bottom  (PI.  Ill,  fig.  3)  is  held  together  by  bolts.'  The 
tackle  attached  to  the  derrick  in  part  consists  of  a  pulley  block  fixed 
to  the  top  of  the  pole  as  shown  in  Plate  III,  figure  2.  A  1-inch  to 
f-inch  rope  is  attached  to  this  pulley,  pa>sing  through  another  pulley 
block  which  is  free,  and  thence  back  through  the  fixed  oik4  (see  PI. 
VII).  The  rope  used  should  be  about  three  times  the  length  of  the 
pole.  When  the  derrick  is  to  be  moved  from  one  tree  to  another  the 
free  pulley  should  be  hooked  to  a  rope  or  ring  on  the  standard,  the 
rope  pulled  taut,  and  the  free  end  tied  temporarily,  as  shown  in 
Plate  III,  figure  3.  This  will  prevent  the  rope  from  becoming 
twisted.  The  movable  pulley  should  have  a  hook  at  the  bottom 
by  which  it  can  be  attached  to  a  ring  on  the  tent.  A  guy  rope  several 
feet  longer  than  the  derrick  is  fastened  at  the  top  of  each  upright 
and  is  used  in  its  manipulation. 

THE  m'taddkn  MACHINE. 

Mr.  C.  E.  McFadden,  of  Fullerton,  CaL,  has  devised  an  elaborate 
and  ingenious  machine  for  placing  tents  on  trees.  A  picture  of  this 
machine  is  shown  in  Plate  II,  figure  2.  In  brief  it  consists  of  an  iron 
framework  mounted  on  a  pair  of  trucks.  At  the  center  of  either  end 
of  the  framework  is  attached  a  long  arm  made  of  iron  tubing. 
These  arms  are  comparable  to  a  pair  of  long  hoisting  poles.  Each  of 
these  arms  is  raised  or  lowered  by  a  system  of  steel  cables  passing 
through  pulleys  attached  to  the  arms  and  two  high  iron  standards. 
These  cables  are  manipulated  by  a  gasoline  engine,  which  also  oper- 
ates another  pair  of  cables  used  to  raise  or  lower  the  tents  to  or  from 


22       '  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

the  end  of  the  poles.  When  ready  to  throw  a  cover,  the  machine  is 
drawn  opposite  the  tree,  the  arms  are  lowered  until  their  ends  are 
suspended  beyond  the  outer  edge  of  the  tree,  cables  are  then  let  down 
from  the  end  pulleys  and  run  through  two  series  of  rings  in  the  tent, 
after  which  the  tent  is  raised  to  the  end  of  the  derricks.  These  rings 
are  so  placed  that  when  the  cable  is  raised  about  one-third  of  the  tent 
is  gathered  up  in  a  series  of  folds.  The  derricks  are  then  erected 
and  the  tent  cables  released,  when  the  tent  will  fall  over  the  tree. 

This  operation  is  quite  rapid  as  well  as  less  wearing  to  the  trees 
and  tents  than  the  use  of  poles  or  derricks.  Although  slower  and  more 
expensive  than  the  use  of  poles  in  covering  small  trees,  it  is  easily 
superior  to  derricks  in  covering  large  ones,  such  as  seedlings,  especially 
where  so  closely  set  that  the  branches  interlace. 

SUPPLY  CART  AND  SUPPLY  WAGON. 

An  apparatus  of  some  sort  is  required  in  carrying  from  tree  to  tree 
the  chemicals  necessary  in  fumigation.  The  idea  of  using  a  two- 
wheeled  pushcart  originated  with  the  San  Bernardino  County  outfits, 
where  this  method  has  been  used  for  several  years.  Observation  of 
its  use  convinced  the  writer  that  in  most  places  the  employment  of 
a  properly,  equipped  handcart  is  the  most  practical  method  available 
for  carrying  the  chemicals.  Extended  effort  has  been  made  to  equip 
such  a  cart  in  a  manner  suitable  for  convenient  use  in  the  field.  The 
result  of  this  effort  is  shown  in  Plate  IV,  figure  1,  the  make-up  of 
which  has  been  so  improved  over  the  original  as  to  resemble  it  but 
little.  As  purchased,  the  cart-bed  consists  of  a  plain  box  fitted  with 
a  two-shaft  handle.  This  handle  is  removed,  and  is  replaced  by  a 
tongue  having  an  enlarged  link-shaped  iron  about  a  foot  long  firmly 
attached  at  the  end.  This  link-shaped  handle  is  very  convenient  in 
field  work.  The  scales  for  weighing  the  chemicals  are  placed  on  a 
platform  above  the  center  of  the  box.  The  ordinary  kind  having  a 
free  scoop  and  using  weights  is  most  convenient;  1,  2,  4,  8,  and  16 
ounce  weights  are  required.  The  cyanid  is  contained  in  a  tin-lined 
box  in  the  rear  half  of  the  cart,  while  the  acid  and  water  are  placed 
in  the  front  end.  A  10-gallon  keg  firmly  attached  in  a  horizontal 
position  to  the  bed  of  the  cart  is  a  very  convenient  receptacle  for  the 
w;iicr.  A  galvanized-iron  basin,  like  that  shown  above  the  keg, 
[laving  an  opening  at  the  bottom  fitting  into  the  bung  of  the  keg, 
makes  ;i  very  satisfactory  funnel  for  filling  the  latter.  The  acid  may 
be  held  in  an  earthenware  jar  or  a  lead-lined  tank,  with  cover  firmly 
attached  to  prevent  slopping. 

By  way  of  a  cover  for  the  earthenware  jar,  a  lead-lined  lid  (fig.  5) 
which  fits  tightly  within  the  top  has  been  used.  At  the  center  of  this 
lid  is  an  opening  about  (>  inches  in  diameter,  around  the  circumference 
of  which  is  attached  a  leaden  tube  which  extends  downward  several 


Bui.  90.  Part  I,  Buieau  of  Entomology,  U.  S.  Dept.  of  Agriculture.  PLATE  III. 


Fig.  1  .—Method  of  Attaching  Tent  to  Hoisting  Pole  by  a  Half  Hitch  of  the 
Rope.  The  Edge  of  the  Tent  is  Double-lapped.  (Original.)  Fig.  2.— Top  of 
Derrick,  Showing  Method  of  Attaching  Pulley  and  Guy  Rope.  Fig.  3.— Base 
of  Derrick,  Showing  Method  of  Constructing  Braces.   (From  Morrill.) 


Bui.  90,  Part  I,  Bureau  of  Entomology,  U.  S.  Dept.  of  Agriculture 


Plate  IV. 


Fig.  2. -Supply  Wagon  Devised  by  C.  E.  McFadden,  of  Fullerton,  Cal.  (Original.) 


FUMIGATION  OF  CITRUS  TREES. 


23 


inches  and  prevents  the  slopping  of  acid  through  the  hole.  A  lead- 
lined  cover  fits  into  the  top  of  this  tube.  This  opening  in  the  cover 
is  for  use  in  filling  the  jar.  Very  few  metals  will  withstand  sulphuric 
acid  without  corroding.  For  this  reason  all  the  common  types  of 
faucets  are  practically  worthless  for  drawing  acid.  In  fact  there  is 
no  faucet  on  the  market  that  is  altogether  satisfactory  for  this  pur- 
pose. A  manufacturing  firm  on  the  Pacific  coast  has  experimented 
extensively  along  this  line,  but  without  any  marked  success.  This 
difficulty  has  been  met  in  an  entirely  practical  manner  by  attaching  a 
three-quarter-inch  copper  or  iron  pine  to  the  lower  side  of  the  jar  and 
regulating  the  flow  of  acid  by  means 
of  a  large  pinehcoek  placed  on  a  short 
piece  of  rubber  tubing  at  the  end  of 
the  pipe  (fig.  5, 1,4,  and  5).  The  flow 
of  acid  is  rapid  and  easy  to  control. 
Pure  rubber  is  most  satisfactory,  and 
a  fresh  piece  should  be  substituted 
about  every  other  night. 

The  water  is  drawn  from  a  faucet. 
In  order  that  this  may  be  drawn  on 
the  same  side  of  the  cart  as  the  acid, 
an  elbow  pipe  of  the  character  shown 
in  Plate  IV.  figure  1 ,  is  required.  The 
faucet  should  have  an  opening  of  about 
three-fourths  inch  to  allow  a  heavy 
flow,  and  should  be  of  such  a  type  that 
a  half-turn  will  give  it  a  full  opening. 

As  fumigation  is  usually  conducted 
at  night,  a  torch  is  placed  on  the 
front  of  the  cart  to  furnish  a  light  by 
which  to  measure  the  acid  and  water: 
another,  on  4he  elevated  platform,  is 
convenient  for  the  man  weighing  the 
cyan  id. 

This  style  of  cart  is  entirely  practicable  for  almost  all  fumigation 
work.  The  chemicals  can  be  measured  quickly  and  accurately  with- 
out any  slopping  of  acid  or  water.  A  glass  graduate  with  a  capacity 
of  16  or  32  ounces,  preferably  the  latter,  is  essential  for  measuring 
acid  and  water.  A  kind  having  elevated  rings  in  the  glass  has  been 
found  most  satisfactory  for  night  work.  The  man  handling  the  acid 
should  wear  rubber  gloves.  Cotton  gloves  are  convenient  for  hand- 
ling cyanid. 

Some  fumigators  have  preferred  to  use  a  horse-drawn  wagon 
equipped  for  carrying  the  chemicals  rather  than  a  handcart.  This 
has  resulted  in  a  number  of  very  original  combinations.    The  most 
67330°— Bull.  90—12  3 


Fig.  5. — Earthenware  acid  jar  with  attach- 
ments for  field  use;  /,  Jar  complete;  ^.inside 
view  of  lead-lined  cover  showing  tul>e  at 
center;  5,  copper  top  for  opening  in  cover; 
U  pinehcoek;  5,  method  of  attaching  iron 
pij>e  to  jar.  and  rubt>er  tube  on  end  of  pipe 
with  pinehcoek  attached.  (Author's  illus- 
tration.) 


24  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

elaborately  equipped  of  these  recent  innovations  was  devised  by  Mr. 
C.  E.  McFadden,  of  Fullerton,  Cab,  and  is  shown  in  Plate  IV,  figure  2. 
Some  of  its  devices  are  very  ingenious  and  well  worthy  of  mention. 
The  large  box  (a)  in  the  middle  of  the  wagon  contains  an  acid  carboy 
whose  neck  can  be  seen  projecting  above  and  into  which  is  inserted 
a  rubber  hose  (b)  which  leads  backward  and  downward  to  the  rear  of 
the  wagon.  Pieces  of  lead  pipe  are  attached  to  the  ends  of  this  hose, 
the  free  end  of  which  (g)  is  equipped  with  a  piece  of  rubber  tubing 
and  cut-off  for  regulating  the  flow  of  acid.  The  water  is  contained 
in  the  barrel  (c)  at  the  front  of  the  wagon.  A  hose  leads  from  the 
bottom  of  this  back  to  the  large  vertical  cylindrical  object  (d)  to  the 
left  of  the  acid  hose  and  adjacent  to  the  wagon  wheel.  This  cylinder 
is  the  graduate  used  for  measuring  water.  The  slender  iron  rod  (e) 
seen  projecting  from  the  top  of  this  cylinder  is  attached  to  a  movable 
float.  This  rod  is  graduated  so  that  each  graduation  is  equivalent  to 
3  ounces  of  water.  The  turning  of  a  valve  at  the  bottom  of  this 
cylinder  allows  the  water  to  flow  in,  raising  up  the  movable  float. 
When  the  graduation  on  the  iron  rod  shows  that  the  cylinder  contains 
the  required  amount  this  valve  is  closed  and  another  opened  which 
directs  the  water  through  a  downward-projecting  pipe  into  the  gen- 
erator placed  beneath.  The  cyanid  is  contained  in  the  box  Qi)  at  the 
rear  of  the  wagon.  The  wagon  is  also  equipped  with  a  thermometer 
and  hydrometer.  The  upper  and  larger  (f)  of  the  two  horizontal 
cylinders  above  the  scales  is  a  rotary  device  for  reading  the  dosage 
schedule.  The  lower  cylinder  (Jc)  contains  a  sheet  of  paper  on  which 
a  record  of  the  dosages  used  is  kept. 

GENERATING  VESSELS. 

In  California  earthenware  vessels  of  the  type  shown  in  figure  12 
(p.  75)  are  made  especially  for,  and  are  almost  exclusively  used  in,  gen- 
erating the  gas.  They  are  sold  without  a  cover.  The  average  capacity 
is  2  gallons,  although  a  1  J-gallon  size  is  sometimes  used  for  small  trees 
and  a  3-gallon  size  for  very  large  ones.  A  H-gallon  generator  will 
serve  for  a  dosage  of  about  15  ounces  of  cyanid  without  boiling  over, 
a  2-gallon  generator  for  approximately  20  ounces,  and  a  3-gallon  one 
for  about  30  ounces,  provided  the  cyanid  is  in  average-sized  lumps 
and  not  powdered.  Where  dosages  larger  than  30  ounces  are  required, 
use  two  generators,  or  three  if  necessary, 

GENERAL  PROCEDURE. 

The  process  of  fumigation  consists  of  covering  trees  with  cloth  tents 
and  generating  beneath  them  a  very  poisonous  gas  called  hydrocyanic- 
acid  gas.  As  previously  mentioned,  sheet  tents  exclusively  are  used 
in  California.    After  exposing  ;i  tree  to  the  gas  for  a  definite  time, 


FUMIGATION  OF  CITRUS  TREES. 


25 


usually  an  hour  or  thereabouts,  the  tent  is  removed  to  the  next  tree 
and  the  process  repeated.    The  work  is  carried  on  at  night. 

Let  it  be  supposed  that  a  man  owning  an  orchard  of  several  acres 
has  made  arrangements  to  have  this  fumigated.  Before  entering  on 
the  actual  work  certain  preliminaries  are  attended  to.  The  fumigator 
prefers  and  usually  requires  that  the  orchard  shall  have  been  culti- 
vated recently  so  that  the  ground  shall  be  clean  and  smooth.  This 
condition  of  the  soil  is  not  only  an  advantage  to  the  fumigator  but  to 
the  grower  as  well,  for  in  loose  level  soil  the  tents  will  lie  closer  to  the 
ground  and  thus  allow  less  escape  of  gas  underneath  than  would  be 
possible  on  weedy  or  roughly  furrowed  land.  The  moving  of  the 
chemical  cart  or  wagon  is  also  more  difficult  on  the  rougher  ground. 

An  outfit  usually  consists  of  about  30  tents.  Before  placing  the 
outfit  in  the  held  the  fumigator  makes  a  survey  of  the  orchard  in 
order  to  determine  in  what  manner  the  tents  can  be  used  to  best 
advantage.  This  depends  on  such  considerations  as  the  arrangement 
of  the  trees,  the  length  of  the  rows  in  different  directions,  slope  of  the 
soil,  whether  irrigation  furrows  are  present,  location  of  water  supply, 
and  similar  factors.  Having  decided  the  direction  in  which  the  tents 
shall  be  pulledj  the  wagon  which  has  moved  the  outfit  from  the  pre- 
ceding field  to  the  present  one  is  driven  along  the  first  row  to  be 
fumigated  and  a  tent  and  generating  pal  dropped  off  a\  each  tree. 

The  u commissary/ '  <>r  place  where  the  supply  of  chemicals  and  water 
is  located,  should  preferably  be  near  one  end  of  the  row  of  tents. 
The  location  of  the  source  of  water  will,  of  course,  determine  this 
position.  If  no  source  is  bordering  the  field,  barrels  should  be  pro- 
vided for  this  purpose.  The  acid  is  usually  furnished  in  large  iron 
drums.  It  is  convenient  to  remove  the  acid  from  the  drums  into 
10-gallon  glass  carboys  of  the  nature  shown  in  figure  f>.  These  ear- 
boys  are  easy  to  handle  and  two  or  three  hold  enough  for  a  full 
night 's  work. 

Immediately  preceding  the  treatment  the  tent-pullers  unfold  the 
tent  va.ud  have  them  in  position  for  covering  the  trees.  This  position 
should  be  with  one  end  facing  the  tree  on  the  side  away  from  the 
direction  in  which  they  are  to  be  moved.  Covering  the  trees  is 
commenced  at  one  end  of  the  row.  Two  poles  of  the  character 
described  on  pages  20-21  are  required,  one  for  either  side  of  the  tree. 
If  rings  are  in  the  tents  the  ends  of  the  poles  are  attached  to  the 
rings.  However,  it  is  very  much  easier  and  more  satisfactory  not 
to  u^e  rings  on  tents  manipulated  by  poles,  but  to  double-lap  the 
edge  of  the  tent  over  the  end  of  the  pole  and  attach  it  by  a  half  hitch 
of  the  pulling  rope  (Plate  III,  fig.  1).  This  is  quickly  done,  does 
not  subject  the  tent  to  undue  wear,  and  prevents  detaching,  as 
sometimes  occurs  with  rings,  but  the  greatest  advantage  is  that  the 
distance  between  the  poles  can  be  gauged  in  accordance  with  the 


26  HYDROCYANIC- ACID  GAS  FUMIGATION  IX  CALIFORNIA. 


width  of  the  tree,  which  frequently  makes  tent  pulling  much  easier 
than  when  the  poles  are  more  broadly  separated.  The  tent  should 
always  be  moved  in  the  direction  of  the  strips  of  cloth  so  as  to  prevent 
pulling  the  seams  apart. 

The  successive  stages  in  the  covering  of  a  tree  are  shown  in  Plate 
V.  In  brief,  they  are  as  follows:  When  the  tops  of  the  two  poles 
have  been  attached  to  the  edge  of  the  tent  the  width  of  the  tree 
distant  from  each  other,  the  bottoms  are  placed  at  the  sides  of  the 
tree  opposite  the  trunk,  as  shown  in  Plate  V,  figure  1 .  Each  tent- 
puller  then  places  one  foot 
on  the  end  of  his  pole  to  pre- 
vent it  from  slipping  and 
pulls  on  the  guy-rope,  thus 
raising  the  upper  end  of 
the  pole  and  the  tent  (see 
Plate  V,  fig.  2).  When 
erected  to  such  an  angle 
that  the  poles  no  longer 
slip,  the  puller  removes  Iris 
foot  from  the  bottom  and 
runs  away  from  the  pole  so 
as  to  secure  a  greater  lever- 
age on  the  rope  (see  Plate 
V,  fig.  3).  The  direction 
of  the  pulling  should  be 
not  only  forward  but  also 
somewhat  to  the  side  so 
as  to  keep  the  tent  taut 
between  the  ends  of  the 
poles  and  thus  prevent  it 
from  being  caught  in  the 
top  of  the  tree  by  sagging. 
After  covering  the  tree  the 
edges  should  be  kicked  in- 
ward so  that  the  tent  hangs  straight  from  the  tree  to  the  ground, 
thus  preventing  unnecessary  space  underneath  and  making  the  tent 
lie  close  to  the  soil. 

The  removal  of  tents  from  one  tree  onto  another  is  done  directly 
without  first  having  to  pull  them  off  onto  the  ground.  In  fact,  it  is 
easier  to  draw  a  tent  off  of  one  tree  onto  another  than  to  raise  it  from 
the  ground  onto  the  tree.  Attach  the  poles  to  the  edge  of  the  tent 
as  previously  explained.  The  poles  can  then  be  laid  flat  on  the 
ground,  as  in  Plate  VI,  figure  la,  or  the  end  with  the  tent  attached 
raised  up  and  leaned  against  the  tented  tree,  as  shown  in  Plate  VI, 


Fig.  6  —Carboy  with  handles  attached  to  facilitate  pour- 
ing the  acid  and  carrying  the  carboy.  (Author's  illus- 
tration.) 


Bui.  90,  Part  I.  Bureau  of  Entomology,  U. 


S.  Dept  of  Agriculture. 


Plate  V. 


Bui.  90,  Part  I,  Bureau  of  Entomology,  U.  S.  Dept.  of  Agriculture. 


Plate  VI. 


FUMIGATION  OF  CITRUS  TREES. 


27 


figure  16.  The  second  step  in  the  procedure  is  shown  in  figure  2  of 
Plate  VI.  while  the  remaining  steps  are  the  same  as  in  Plate  V, 
figures  b  •").  and  6. 

In  covering  very  large  trees  derricks  of  the  nature  described  on 
page  21  are  used.  Four  men  are  required  for  their  manipulation, 
which  is  shown  in  Plate  VII.  A  derrick  is  raised  to  a  nearly  upright 
position  at  eacli  side  of  the  tree  (Plate  VII,  figs.  1-3),  leaning  at  a 
slight  angle  backward  and  held  in  this  position  by  the  guy-rope 
attached  to  its  top  (Plate  VII,  fig.  3).  The  movable  pulley  of  each 
derrick  is  then  attached  to  a  ring  in  the  tent  (Plate  VII,  fig.  3)  and 
pulled  up  to  the  top  of  the  derrick,  where  it  is  held  (Plate  VII,  fig.  4). 
By  pulling  on  the  guy-rope  the  derrick  is  caused  to  fall  forward, 
drawing  the  tent  over  the  tree. 

CALCULATING  THE  DOSAGE. 

Having  covered  the  trees,  the  next  requirement  is  the  amount  of 
chemicals  to  use.  or  the  dosage.  The  dosage  is  the  most  important 
consideration  in  the  gas  process.  It  varies  not  only  with  the  size  of 
the  tree  hut  also  with  the  character  of  insect  to  be  destroyed.  Spe- 
cific recommendations  of  dosage  for  the  principal  insects  injurious 
to  citrus  trees  in  California  are  given  eUew  here  in  this  bulletin 
(pp.  51-61). 

The  Gist  requirement  in  calculating  the  dosage  for  a  tree  is  to 
compute  the  cubic  contents  inclosed  by  the  tent  when  in  position 
over  the  tree.  Although  most  citrus  trees  possess  a  certain  general 
similarity  in  shape,  they  are  nevertheless  somewhat  irregular,  no  two 
ever  being  identical  in  all  respects.  This  irregularity  renders  it 
impracticable  to  determine  the  contents  to  w  ithin  a  cubic  foot  or  so 
of  its  actual  volume;  yet  it  can  be  approximated  with  a  sufficient 
degree  of  accuracy  for  such  practical  work  as  fumigation.  In  order 
to  calculate  the  cubic  contents  of  an  object  it  must  be  considered  as 
shaped  like  some  regular  geometrical  figure  or  figures.  The  figure 
which  most  closely  approximates  in  shape  an  orange  or  lemon  tree 
before  it  has  been  pruned  is  a  cylinder  surmounted  by  a  hemisphere, 
and  in  computing  the  volume  of  citrus  trees  they  are  considered  to 
be  of  this  shape. 

If  the  height  and  width  of  a  tree  covered  with  a  tent  is  known,  it 
is  a  comparatively  simple  matter  to  calculate  its  contents.  In  the 
past  work  in  California  the  dosage  has  been  based  upon  these  two 
measurements.  After  a  tree  is  covered  with  a  tent  it  is  a  matter  of 
some  difficulty  to  determine  its  height  and  width.  By  using  as  fac- 
tors the  distance  around  the  bottom  of  the  tent  and  the  longest  dis- 
tance over  the  top  of  the  tent  we  arrive  at  a  more  practicable  method 
by  which  t<>  compute  the  cubic  contents  of  a  given  tree.    Using  these 


28       '   HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


measurements  as  a  basis  the  writer  has  invented  a  formula  1  by 
means  of  which  the  cubic  contents  of  a  tree  may  be  computed. 

To  avoid  computation  work  in  the  field,  in  so  far  as  possible,  the 
writer  has  formulated  a  table  approximating  the  cubic  contents  of 
trees  of  different  dimensions,  which  is  sufficiently  extensive  to  include 
any  citrus  tree  in  southern  California.  During  this  investigation  no 
tree  has  been  found  whose  dimensions  did  not  fall  within  the  limits 
given  in  this  table.  The  distance  around  and  over  a  given  tree  being 
known,  the  table  will  show  the  approximate  cubic  contents  of  the 
tented  tree.  The  dosage  can  then  be  applied  in  proportion  to  the 
contents  and  at  any  strength  desired. 

A  lemon  tree,  after  being  pruned,  is  flat  on  the  top.  Therefore 
the  geometrical  figure  which  is  applicable  to  an  orange  orunpruned 
lemon  tree  can  not  be  considered  as  applicable  to  a  pruned  or  flat- 
topped  lemon  tree.  The  figure  which  approximates  the  latter  is  a 
cylinder.  Now  it  so  happens  that  the  contents  of  a  cylinder  having 
certain  dimensions  over  its  top  and  around  its  bottom  are  almost 
the  same  as  for  a  figure  of  the  same  dimensions  composed  of  a  cyl- 
inder surmounted  by  a  hemisphere.  This  is  a  great  advantage,  for 
the  schedule  of  dosage  proposed  for  orange  trees  may  also  be  used 
for  all  lemon  trees,  thus  obviating  the  necessity  of  preparing  two 
different  schedules. 

SECURING  THE  MEASUREMENTS  AROUND  AND  OVER. 

The  distance  around  the  bottom  of  a  tent  is  easily  secured  by  the 
use  of  a  tapeline,  or  by  pacing.  The  distance  over  the  top,  however, 
was  much  more  difficult  to  determine  until  Dr.  A.  W.  Morrill,2  in 
the  course  of  his  work  for  the  Bureau  of  Entomology  against  the  citrus 
white  fly  (Aleyrodes  citri  R.  &  H.)  in  Florida,  invented  a  method  of 
marking  tents  for  this  purpose.  The  Morrill  method  renders  the 
securing  of  the  distance  over  the  top  of  the  tent  as  easy  as  that 
around  the  bottom. 

1  Prof.  Woodworth  (Bui.  152,  Univ.  of  Cal.  Agr.  Exp.  Sta.,  p.  5, 1903)  was  not  only  the  first  to  suggest  the 
measurements  around  t  he  bottom  and  over  the  top  of  tented  trees,  but  also  was  the  first  to  propose  a  for- 
mula for  obtaining  the  contents  of  tented  trees  based  on  a  knowledge  of  these  distances.  An  analysis  of 
this  formula  during  the  early  part  of  the  writer's  field  work  proved  that  it  was  inaccurate,  thus  necessitat- 
ing the  determination  of  a  new  formula.  The  writer  has  worked  out  a  formula  based  on  the  two  measure- 
ments above  mentioned.   It  is  as  follows: 

C2/Q_C(3^-4)\ 
4*  \  2         12*  / 
In  this  formula  C  equals  the  circumference  of  the  tree. 

O  equals  the  distance  over  the  top  of  the  tree. 

IT  a  person  works  out  and  notes  down  in  a  chart  the  values  of  ^  and  ^  for  different  values  of  C  of 
which  lie  Is  apt  to  make  common  use,  it  is  possible  by  its  use  in  connection  with  the  formula  to  determine 
the  contents  of  irops  with  fair  rapidity. 

2  Bill.  7<i,  Bur.  Ent.,  U.  S.  Dept.  Agr.,  pp.  31-34, 1908. 


Bui.  90,  Part  I,  Bureau  of  Entomology,  U.  S  Dept.  of  Agriculture 


Plate  VII. 


FUMIGATION  OF  CITRUS  TREES. 


29 


THE   MORRILL  METHOD. 


In  figure  7  is  shown  an  outline  of  a  regulation  fumigating  tent 
marked  after  the  Morrill  system.  Three  parallel  lines  and  one  line 
at  right  angles  to  them  are  indicated  on  the  tent."  The  middle  one 
of  the  three  parallel  lines  passes  through  the  central  point  in  the  tent 
canvas,  running  lengthwise  of  the  central  section  or  strip  of  which 
the  tent  is  made  and  passing  over  the  top  of  the  tent  from  the  edge 


Fig.  7.— Outline  of  a  shoot  fumigation  tont  marked  according  to  the  Morrill  method.  (Author's  illustration.) 


on  one  side  to  the  edge  on  the  opposite  side;  these  lines  also  run  in 
the  direction  in  which  the  tent  should  be  pulled  on  or  off  a  tree. 
Beginning  at  the  center  these  lines  are  graduated  in  feet  toward 
either  edge  of  the  tent,  after  the  manner  shown  in  the  diagram. 
For  tents  above  36  feet  (average  size)  it  is  unnecessary  to  commence 
the  graduation  nearer  than  5  feet  from  the  center  of  the  canvas. 
When  one  of  these  lines  is  over  the  middle  of  the  tree  the  distance 
over  can  be  calculated  by  merely  adding  together  the  two  numbers 
on  the  opposite  sides  of  the  tent  where  the  edge  touches  the  ground. 


30  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

For  instance,  suppose  that  on  the  line  over  the  center  of  the  tree  12 
is  nearest  the  ground  on  one  side  and  15  on  the  other.  The  distance 
over  the  center  of  this  tree  would  be  the  sum  of  these  numbers,  which 
is  27  feet.  With  the  lines  graduated  after  this  manner  it  makes  little 
difference  in  determining  the  distance  over  the  top  of  the  tree  whether 
or  not  the  geometrical  center  of  the  tent  is  at  the  center  of  the  tree, 
the  single  requirement  being  that  some  part  of  one  of  the  graduated 
lines  approximates  the  center  of  the  tree. 

The  two  lines  running  parallel  to  this  central  line-  should  be  about 
4  feet  distant  from  it  in  the  larger  fumigating  tents.  The  reason  for 
using  these  auxiliary  lines  is,  that  in  practice  the  center  of  the  tent 
is  very  often  pulled  considerably  to  one  side,  especially  in  covering 
small  trees.  If  the  middle  line  does  not  fall  immediately  over  the 
center  of  the  tree,  one  of  the  other  two  lines  is  quite  likely  to  do  so, 
and  that  one  should  be  used  in  obtaining  the  distance  over. 

The  cross  line  running  at  right  angles  to  the  three  parallel  lines  also 
passes  through  the  center  of  the  tent  and  is  marked  like  the  others. 
The  idea  of  this  cross  line  is  that  in  case  of  an  irregularly  shaped 
tree  the  distance  over  can  be  taken  in  two  different  directions  and  the 
average  taken  for  use  in  determining  the  cubic  contents.  For  experi- 
mental purposes  with  a  few  tents  this  line  is  an  advantage,  but  in 
practical  operations  it  is  unnecessary  and  should  never  be  placed  on 
the  tent,  as  measurement  over  the  top  in  one  direction  is  sufficient. 
The  presence  of  so  many  lines  tends  to  confuse  the  operator. 

Having  calculated  the  volume  of  a  tree  from  the  two  measurements, 
around  and  over,  it  is  possible  to  dose  the  tree  at  any  strength  per 
unit  volume  desired.  When  the  dosage  has  been  determined  the 
chemicals  are  measured  out  and  placed  underneath  the  tented  trees. 

THE  OLD  METHOD  OF  PROCEDURE. 

When  this  investigation  was  started,  a  system  of  fumigation  was 
used  exclusively  in  which  the  dosage  given  the  trees  was  based 
entirely  on  guesswork.  The  estimator,  who  ordinarily  is  the  man  in 
charge  of  the  outfit,  starts  out  in  an  orchard  equipped  with  a  blank 
schedule  sheet  of  cross-section  paper.  He  walks  between  two  rows 
of  trees,  jotting  down  in  the  corresponding  squares  of  the  schedule 
sheet  the  dosage  which  lie  believes  the  trees  should  receive.  This 
dosage  is  based  on  his  eyesight  supported  by  his  past  experience. 
If  lie  is  a  careful  scheduler,  he  will  look  at  the  trees  from  different 
sides  before  indicating  the  dosage,  as  trees  are  sometimes  more  com- 
pile! on  one  side  than  on  another.  Less  careful  men  set  down  the 
dosage  lor  the  two  rows  of  trees  while  moving  along  as  fast  as  they 
can  walk.  rY\w  writer  has  seen  some  schedulers  walk  through  the 
orchard  ;il  a  rapid  pace,  taking  four  rows  at  a  time. 


FUMIGATION  OF  CITRUS  TREES. 


31 


The  inaccuracy  of  such  a  method  is  at  once  apparent.  Measure- 
ment- made  after  many  estimators  have  shown  that  the  most  careful 
are  very  irregular  in  their  scheduling.  No  one  has  been  found  who 
does  not  at  times  vary  as  much  as  50  per  cent  in  dosage  estimates 
for  trees  containing  exactly  t ho  same  cubic  contents  after  being 
covered  witli  tents.1  Tins  variation  in  the  scheduling  of  an  indi- 
vidual fumigator  is  not  all,  but  the  general  average  dosage  used 
by  one  man  has 
frequently  been 
one-fourth  to  one- 
half  more  and 
sometimes  even 
twice  that  used  by 
another  for  the 
very  -nine  Insect. 

This  chart  of  dos- 
age for  t  he  t  roes  in 
an  orchard  is  taken 
into  t  he  Held  at 
night.  Before  dos- 
ing a  row  of  t  roes 
t  he  common  met  h- 
od  is  to  first  meas- 
ure out  the  dosages 
for  the  t  ices  in  this 
row  into  small  cans 
and  pitchers,  which 
a  re  p 1  a  cod  i  n  a 
hand  tray,  as  show  n 
in  figure  S.  This 
tray  is  then  carried 
from  one  t  re  4  to  the 
next  down  the  row  . 
The  water  is  carried  in  a  pail 
instruments  used  for  measurim 


Man  carrying  tray  and  water  bucket  as  practiced  under  old 
BVStem Of  fumigation.    (Author's  illustration.) 


and  measured  at  each  tree.  The 
the  water  have  been  found  to  vary 
all  the  way  from  graduated  dippers  to  quart  pitchers  or  old  tin  cans, 
ruder  this  old  method  the  general  results  secured  by  a  few  of  the  more 
careful  and  expert  fumigators  have  been  fairly  good.  However,  the 
work  in  the  majority  of  cases  has  been  irregular  and  poor.  This  old 
Bystem  is  rapidly  sinking  into  disuse,  being  replaced  by  an  improved 
procedure  which  has  resulted  from  the  present  investigations. 


I  Sec  Bui.  79,  Bur.  Ent.,  U.  S.  Dept.  Agr.,  pp.  23-24.  1909. 


32  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

AN  IMPROVED  SYSTEM  OF  FUMIGATION. 

After  becoming  acquainted  with  the  chaotic  condition  of  the  fumi- 
gation practice  as  it  existed  at  the  commencement  of  this  investigation; 
it  was  very  evident  that  some  system  should  be  perfected  which 
would  entirely  eliminate  the  guess  features  and  provide  for  a  calcu- 
lation of  the  dosage  based  directly  on  the  size  of  the  tree.  As  the 
result  of  extended  observation,  experimentation,  and  devising,  a 
system  of  fumigation  having  decided  advantages  over  the  old  method 
was  introduced  into  California  during  the  month  of  July,  1908.  This 
system  is  by  no  means  original,  but  is  largely  the  result  of  utilizing, 
correcting,  correlating,  systematizing,  and  making  entirely  practical 
the  best  methods  and  ideas  which  either  had  been  in  practice  or  had 
been  suggested  in  the  State  before  this  investigation  was  begun.  The 
method  of  procedure  was  copied  largely  after  that  of  the  San  Ber- 
nardino County  outfits,  while  the  method  of  calculating  the  dosage 
was  suggested  by  Prof.  Woodworth,  of  the  University  of  California, 
and  made  practical  by  the  adoption  of  the  Morrill  method  of  marking 
tents.  A  method  quite  similar  in  general  features  to  the  California 
improved  system  was  introduced  into  Florida  by  Dr.  A.  W.  Morrill, 
during  the  winter  of  1 907-8.  Each  of  these  systems  was  in  the  process 
of  evolution  at  the  same  time,  yet  almost  entirely  independent  of 
each  other.  To  Dr.  Morrill,  however,  greatest  credit  is  probably  due 
for  the  present  advance  in  procedure,  as  the  inventing  of  his  method 
of  marking  tents  was  the  turning  point  between  impractical  and 
practical  scientific  field  calculations. 

The  tents  should  be  marked  after  the  Morrill  method  described  on 
pages  29-30.  Only  three  parallel  lines  are  used,  the  crossline  being  not 
only  unnecessary  but  a  disadvantage  in  practical  work.  This  sup- 
plies an  easy  and  rapid  means  of  determining  the  distance  over  the 
top  of  the  tree.  The  distance  around  the  tented  tree  can  be  measured 
accurately  by  pacing  if  this  is  done  in  a  careful  manner.  Experience 
with  men  on  the  outfit  used  in  this  investigation  as  well  as  on  some 
of  the  practical  outfits  which  first  adopted  the  improved  system 
demonstrated  that  after  some  practice  in  pacing  around  tents  some 
men  could  so  regulate  their  pace  as  to  be  sufficiently  accurate  for 
practical  purposes.  This  resulted  in  at  first  advising  the  method  of 
pacing  to  secure  this  distance.  The  broad  adoption  of  this  improved 
sysl  em,  as  well  as  the  frequent  changes  that  take  place  in  the  personnel 
of  a  crew,  resulted  in  the  pacing  being  assigned  to  various  types  of 
men,  sonic  of  whom  have  been  known  to  be  hasty  and  careless. 
Another  discouraging  feature  is  that  the  tents  are  very  often  not 
properly  kicked  in  around  the  bottom  of  the  tree,  which  interferes 
with  accurate  pacing.  To  eliminate  the  possibility  of  irregularity 
due  to  I  he  above  causes  il  is  now  advised  that  pacing  be  discontinued 


FUMIGATION  OF  CITRUS  TREES. 


33 


and  that  the  distance  around  the  tent  be  secured  by  means  of  a  tape. 
To  meet  this  requirement,  a  scheme  has  been  devised  by  Messrs. 
Griffin  and  Gray,  of  YVhittier,  CaL,  which  renders  the  securing  of  the 
distance  around  the  tent  not  only  absolutely  accurate  but  also  more 
rapid  and  easy  than  by  pacing.  The  apparatus  consists  (1)  of  a 
Straight  iron  rod  3  or  4  feet  long  and  about  one-half  inch  in  diameter, 
having  the  lower  end  sharpened  while  the  upper  end  is  made  in  the 
form  of  a  loop,  and  (2)  a  strong  tapeline  haying  a  snap  at  one  end  by 
which  it  is  fastened  to  the  loop  of  the  iron  rod. 

To  secure  the  distance  around  a  tent  the  iron  rod  is  stuck  into  the 
ground  at  one  end  of  the  marked  line  on  the  tent  which  runs  over  the 
top  of  the  tree  (PI.  V,  fig.  6).  The  operator  then  moves  around 
the  tree,  allowing  the  tape  to  slip  through  his  hand  as  he  moves. 
When  he  has  obtained  the  distance  around  he  drops  the  tape,  takes 
the  iron  rod,  with  the  tape  attached,  to  the  next  tree,  and  continues 
as  before.  In  this  manner  the  operator  is  required  to  move  only 
once  around  each  tree.  This  method  is  entirely  practical,  as  proved 
by  experience,  and  in  having  their  work  done  the  growers  should 
demand  its  use.  It  reduces  variation  resulting  from  the  work  of 
careless  operators  to  a  minimum.  From  these  two  measurements 
(the  distance  around  and  the  distance  over)  it  is  possible  to  approxi- 
mate the  cubic  contents  of  the  t  ree  and  thereby  calculate  the  dosage. 
This  might  be  done  in  the  field  and  the  t  ices  then  dosed  in  pro- 
portion to  the  contents.  However,  the  time  required  for  the  cal- 
culation of  the  dosage,  even  after  determining  the  cubic  contents 
of  the  tree,  would  not  only  prevent  rapid  field  work  and  allow  an 
opportunity  for  error,  but  would  cause  a  lack  of  uniformity  in  dosage 
from  the  consideration  of  the  cubic  contents  alone,  as  will  be  explained 
later.  This  difficulty  has  been  obviated  by  preparing  a  dosage 
schedule  from  which  the  required  dosage  may  be  calculated  without 
any  figuring  as  soon  as  the  measurements  of  the  tree  are  known. 

LEAKAGE  OF  (IAS.1 

One  of  the  most  important  questions  relating  to  the  proper  dosage 
in  fumigation  is  that  of  leakage  of  gas  through  the  tent.  In  fact, 
with  the  present  character  of  tenting,  where  the  gas  has  usually  all 
escaped  by  the  end  of  an  hour,  the  dosage  depends  directly  on  the 
amount  of  this  leakage.  In  figures  which  approximate  a  citrus  tree 
in  shape  the  volume  decreases  at  a  more  rapid  rate  than  does  the 
surface.  Computation  shows  that  a  tree  20  feet  around  by  V?  feet 
over  has  0.86  of  a  square  foot  of  tent  surface  for  each  cubic  foot  of 
gas  within  to  escape  through,  whereas  a  tree  79  by  54  feet  has  only 
0.22  of  a  square  foot  of  tent  surface  for  each  cubic  foot  of  gas  to 
escape  through.    This  would  mean  that  there  is  about  four  times  as 


i  See  Bui.  79,  Bur.  Ent.,  U.  S.  Dept.  Agr.,  p.  47,  1909. 


34  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

great  an  opportunity  for  leakage,  or  that  the  leakage  would  be  approx- 
imately four  times  as  rapid  in  the  smaller  tent  as  in  the  larger  one. 
There  can  be  little  doubt  that  the  leakage  of  gas  through  most  of  the 
tenting  materials  used  in  this  State  is  nearly  in  accordance  with  these 
figures.  In  order  to  secure  uniformity  of  results  this  leakage  must 
be  taken  into  consideration  and  small  trees  must  receive  more  cyanid 
to  100  cubic  feet  than  the  larger  trees.  The  correctness  of  the  fore- 
going statement  has  been  repeatedly  demonstrated  during  the  work 
in  the  field.    Reference  to  the  leakage  of  gas  through  tents  was  first 


D/SZ4A/C£T  /ItfOCA/D,    /AS  F£T£T. 


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Fig.  9.— Dosage  schedule  No.  1,  for  potassium  cyanid.  (Original.) 


made  by  Prof.  Woodworth,1  and  in  a  recent  publication  Dr.  A.  W. 
Morrill 2  has  given  it  a  very  thorough  treatment. 


DOSAGE  SCHEDULE. 

After  having  performed  a  large  number  of  experiments  against  the 
purple  scale,  and  determined  the  dosage  required  to  destroy  this  in- 
sect on  different  sized  trees,  the  writer  utilized  these  results  in  pre- 
paring  a  schedule  of  dosage  to  be  used  in  fumigation.  This  schedule 
(see  fig.  (.»)  has  been  designated  "dosage  schedule  No.  1."  The 
dosages  used  in  this  schedule  are  entirely  original  with  the  writer, 
being  the  result  of  experimental  work  supplemented  by  calculations 
as  explained  in  the  following  paragraphs.    After  the  dosages  were 


1  Sc.-  Bui.  i.V2,  Univ.  of  Oil.  Agr.  Exp.  Sta.,  1903.       2  See  Bui.  7«,  Bur.  Ent.,  17.  S.  Dept.  Agr.,  1908. 


FUMIGATION  OF  CITRUS  TREKS. 


35 


determined  thcv  were  introduced  into  a  chart  of  the  same  general 
form  as  that  used  by  Dr.  Morrill.  Such  a  chart  form  baa  been  known 
in  scientific  work  for  many  years  and  was  first  introduced  into  fumi- 
gation scheduling  by  Prof.  Woodworth.1 

An  average-sized  orange  tree,  one  41  feet  in  circumference  by  28 
feet  over,  was  taken  afl  a  basis  in  the  preparation  of  the  schedule. 
The  cubic  contents  of  the  tree  was  determined  and  a  dosage  calcu- 
lated which  would  give  it  1  ounce  to  each  100  cubic  feet.  Trees  of 
other  dimensions,  both  larger  and  smaller,  were  then  considered  and 
their  contents  determined.  In  working  out  the  dosage  for  these  other 
trees  not  only  was  the  cubic  contents  taken  into  consideration  but 
also  the  rate  of  leakage  as  compared  with  that  of  the  tree  41  by  2X 
feet  in  size.  Trees  which  were  smaller  than  this  would  have  a  greater 
proportional  leakage  rate  while  the  larger  ones  would  have  less,  as  ex- 
plained on  pages  \.  In  securing  the  dosage  for  various  trees,  those 
smaller  than  II  by  2N  were  given  sufficient  cyanid  in  excess  of  1  ounce 
per  100  cubic  feet  to  offset  the  increased  leakage,  while  the  dosages  for 
larger  trees  were  proportionately  decreased  below  the  L -ounce  rate. 

This  allowance  for  leakage  BO  modified  the  schedule  that  some  of  the 
smaller  trees  were  receiving  in  excess  of  1  \  ounces  per  100  cubic  feet, 
while  t  rees  as  large  as  60  by  4  1  were  recei\  ing  only  about  t  hree-fourt  hs 
of  an  ounce  to  t  he  same  space.  It  thus  can  be  seen  t  hat  each  dosage 
was  worked  out  independently  and  so  correlated  to  the  other  dosages 
that  when  placed  in  schedule  No.  1  the  ultimate  result  was  that  of  a 
schedule  which  should  approximate  uniform  results  throughout. 

How  to  use  the  chart.-  Referring  again  to  figure  <>,  the  top  line  of 
numbers,  commencing  at  10  and  continuing  up  to  OX,  represents  the 
distance,  in  feet,  around  the  bottom  of  the  tent.  The  outer  vertical 
columns  of  numbers,  on  either  side,  commencing  at  10  and  increasing 
regularly  to  4(.),  represent  the  distance,  in  feet ,  over  the  top  of  the  tent. 
The  dosage  of  a  tree  of  known  dimensions  is  found  in  that  square  where 
the  vertical  c  >lumn  headed  by  the  distance  around  the  tree  intersects 
the  horizontal  line  of  figures  corresponding  to  the  distance  over.  For 
instance,  in  the  case  of  a  tree  40  feet  around  by  28  feet  over,  in  the  top 
line  of  numbers  40  is  next  after  the  third  heavy  vertical  line.  The 
dosages  computed  for  trees  40  feet  around  are  to  be  found  in  the  ver- 
tical column  headed  by  this  number,  which  commences  with  7  and 
ends  with  16.  Then  the  vertical  column  of  large  figures  at  either 
margin  is  followed  down  until  28  is  reached.  All  dosages  computed 
for  t  pees  28  feet  over  are  found  in  this  horizontal  line  of  figures,  which 
commences  with  8  and  ends  at  16.  The  dosage  for  a  tree  40  by 
2S  feet  is  found  at  the  intersection  of  this  line  with  the  vertical  col- 
umn headed  with  40,  that  number  being  11,  the  required  dosage  of 
cyanid  in  ounces.    Before  the  numbers  20,  30,  40,  and  45  in  the  lines 


I  Bui.  162,  Univ.  ofCal.  Apr.  Exp.  Sta.,  p.  6,  1903. 


36  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

at  the  right  and  left  margins  are  to  be  found  blank  spaces,  and  in  the 
horizontal  lines  corresponding  to  these  the  numbers  at  the  top  of  the 
chart  are  repeated  in  that  part  of  the  chart  containing  dosage  figures. 
These  numbers,  repeated  in  this  manner,  make  it  easier  for  the  eye 
to  locate  with  certainty  the  dosage  figures  sought.  In  the  chart  used 
by  the  writer  the  figures  representing  distances  around  and  over  are 
printed  in  red.  The  lines  bounding  these  columns  of  figures  are  also 
red.    All  the  rest  of  the  lines  and  figures  are  black. 

The  writer  does  not  maintain  that  this  table  is  accurate  to  the 
minutest  part  of  an  ounce  for  every  dosage,  but  that  such  variations 
as  do  exist  are  so  small  that  in  practical  work  in  the  field  the  results 
in  killing  scale  insects  will  be  found  uniformly  satisfactory  through- 
out. Two  years  of  experience  with  the  outfits  belonging  to  this 
investigation,  as  well  as  with  many  practical  outfits,  in  which  work 
thousands  of  acres  have  been  fumigated,  have  proved  that  this 
belief  is  well  founded. 

It  is  a  common  practice  with  tent-pullers  in  covering  small  trees 
to  kick  in  the  edge  of  the  tents  little  if  any  so  as  to  prevent  as  much 
as  possible  the  generator  as  well  as  the  escaping  gas  from  coming  in 
contact  with  the  tent.  This  common  practice  leaves  much  more 
space  under  the  tent  and  incidentally  makes  more  leakage  surface  in 
small  trees  than  was  provided  for  in  the  original  chart.  (Bulletin 
No.  79,  fig.  28,  p.  65.)  To  correct  this  feature  of  the  fumigation 
practice,  the  original  chart  has  been  revised  by  increasing  the  smaller 
.dosages  to  the  extent  which  in  field  practice  has  been  found  necessary. 
In  this  revised  chart  the  half  ounces  are  not  used  as  in  the  original. 
Field  experience  has  taught  that  it  is  desirable  to  have  schedules  as 
.simplified  as  possible.  The  writer  would  now  advise  all  fumigators 
to  discard  all  old  schedules  and  to  use  the  revised  one  entirely.  The 
dosage  strength  on  which  this  schedule  was  based  was  calculated  for 
use  against  the  purple  scale.  However,  this  does  not  imply  that  it  may 
not  be  used  against  other  insects;  in  fact,  the  greatest  advantage 
of  it  is  that  it  can  be  manipulated  so  as  to  meet  the  requirements  for 
use  against  any  insect.  The  schedule  in  its  original  form  is  not  recom- 
mended for  use  against  all  insects  under  all  conditions,  as  many  have 
believed.  Some  of  the  scale  pests  frequenting  citrus  trees  require 
a  heavier  dosage  for  their  destruction  than  others.  The  first  point 
to  be  determined  is  the  strength  of  gas  required  for  a  particular  pest 
under  its  special  conditions.  When  this  is  known,  then  schedule 
No.  I  can  be  manipulated  to  meet  the  requirements,  provided  it  is 
nol  already  of  the  proper  strength.  This  change  is  secured  by 
increasing  or  decreasing  all  the  dosages  throughout  schedule  No.  1  at 
the  same  rale,  i.  c,  J,  J,  ],  etc.  The  resultant  schedule  will  be  one 
"I"  uniformity  even  as  is  the  original.  So  in  treating  an  orchard  the 
first  point  to  determine  is  what  speries  of  insect  has  to  be  combated. 


FUMIGATION  OF  CITRUS  TREES. 


37 


Haying  determined  this,  the  dosage  strength  to  use  must  next  be 
learned.  On  this  latter  point  the  writer  has  spent  much  time,  and 
elsewhere  in  this  bulletin  will  be  found  information  as  to  what  dosages 
should  be  used  for  most  of  the  common  scale  pests. 

PROCEDURE. 

Five  men  are  required  to  operate  this  system  to  advantage.  Two 
men  pull  the  tents  and  kick  in  the  edges  around  the  bottom  of  the 
tree.  One  man  takes  the  measurements  of  the  tree,  and  should  also 
empty  the  generator  to  be  used  for  that  tree  and  have  it  in  readiness 
by  the  time  the  supply  cart  arrives.  He  should  empty  the  generator 
with  one  and  t  he  same  hand  at  all  times,  and  with  this  hand  he  should 
never  touch  the  tent.  lie  should  also  be  careful  not  to  slop  any  of  the 
residue  on  his  clothes  or  shoes  le^t  it  bo  rubbed  off  on  the  tent  and  thus 
produce  acid  boles.  The  supply  cart ,  described  on  pages  22-23,  is  most 
convenient  for  carrying  the  chemicals  from  tree  to  tree.  Two  men 
look  after  the  chemicals  -one  measures  the  water  and  acid,  the  other 
weighs  the  cyanid.  The  latter  then  holds  up  t  he  edge  of  the  tent  while 
the  acid  man  places  the  charge  beneath  the  tree.  (See  PL  VIII, 
hg-  2.) 

In  actual  held  practice,  after  the  tent-pullers  have  commenced 
removing  the  tents  the  cart  is  brought  to  one  end  of  the  row  which 
is  to  be  fumigated.  (See  PI.  VIII,  fig.  1.)  The  estimator  secures 
his  measurements  and  calls  them  out  to  the  cyanid  weigher  at  the  rear 
of  the  cart,  who  then  determines  the  dosage  from  a  schedule  which  is 
fastened  to  the  raised  platform.  The  required  amount  of  chemicals 
is  then  measured  and  the  tree  dosed.  While  the  handlers  of  cyanid 
and  acid  are  thus  engaged  the  estimator  has  moved  on  to  the  next 
tree,  secured  his  measurements,  and  holds  his  generator  in  readiness 
when  the  cart  is  brought  up.  This  tree  is  dosed  in  the  same  manner 
as  the  firsl .  and  thus  the  procedure  continues  until  the  entire  row  has 
been  fumigat  »d. 

The  above  procedure  is  such  as  the  writer  has  used  in  the  held  and 
as  has  been  followed  by  most  outfits  using  the  improved  system. 
In  the  procedures  adopted  by  some  other  outfits  there  are  marked 
differences  in  the  duties  of  the  differenl  men. 

ADVANTAGES   UNDER  THIS  SYSTEM. 

This  improved  system  possesses  decided  advantages  over  all 
others.  The  element  of  guess  in  estimating  dosage  and  the  conse- 
quent waste  of  cyanid  under  the  old  method  are  eliminated.  With 
the  use  of  a  known  dosage  strength  certain  definite  and  uniform 
results  occur.  The  chemicals  are  measured  accurately  and  the  most 
economical  proportion  used  at  all  times.  Each  tree  gets  the  dosage 
scheduled  for  it — a  result  which  did  not  always  happen  under  the 
old  method,  owing  to  confusion  of  the  cans  on  the  tray.    The  tent- 


38  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


pullers  seldom  get  more  than  one  or  two  trees  ahead  of  the  cart, 
and  thus  all  trees  receive  the  same  length  of  exposure.  The  work  of 
the  men  who  dose  the  trees  is  easier  than  where  the  heavy  trays  are 
used.  Another  decided  advantage  is  that  the  orchardist  can  at  any 
time  determine  what  character  of  treatment  is  being  given  his  trees 
and  see  that  the  work  is  properly  carried  out.  In  the  past  many 
persons  have  been  prone  to  look  upon  fumigation  as  a  process  that 
is  complex  and  more  or  less  mysterious.  In  some  cases  fumigators 
of  years'  experience  have  encouraged  this  widely  prevailing  opinion, 
so  that  they  might  themselves  be  looked  upon  as  experts  in  a  prac- 
tice difficult  to  understand  and  only  capable  of  being  successfully 
performed  by  men  of  long  experience  and  special  qualifications. 
This  is,  of  course,  erroneous.  The  improved  system  outlined  in 
these  pages  shows  how  simple  the  practice  of  fumigation  may  be 
made.  Careful  men  who  have  never  before  heard  of  fumigation 
can  begin  the  practice  of  this  system  and  are  competent,  after  a 
few  hours  of  instruction,  to  secure  even  better  results  than  were  to 
be  expected  from  the  most  expert  fumigator  in  California  under  the 
old  method.  This  system  reduces  fumigation  to  a  matter  of  simple 
mechanical  operation,  entirely  intelligible  to  the  average  man, 
wherein  the  operator,  to  obtain  the  best  results,  is  required  merely 
to  proceed  according  to  the  formulas  and  directions  given.  Hence, 
orchardists  are  enabled  to  own  their  own  outfits  and  carry  on  their 
work  after  the  most  economical  as  well  as  successful  manner.  Owner- 
ship of  tents  is  to  be  advised  for  several  reasons,  which  are  fully 
explained  elsewhere  in  this  bulletin.  Fumigation  then  can  be  con- 
ducted at  the  most  opportune  time,  good  and  careful  work  is  assured, 
while  the  expense  is  at  a  minimum.  An  orchardist  owning  his  own 
tents  can  keep  his  fruit  clean  at  all  times,  which  otherwise  might  be 
impossible,  because  the  number  of  fumigation  outfits  in  southern 
California  at  the  present  time  is  insufficient  to  cope  properly  with 
the  situation. 

EXPERIEXCE   WITH  THIS  SYSTEM. 

The  introduction  of  this  improved  system  was  made  with  two 
outfits  of  the  Whittier  Citrus  Association,  at  Whittier,  Cal.,  during 
the  latter  part  of  July,  1908,  using  dosage  schedule  No.  1  for  the 
purple  scale.  On  August  15  a  demonstration  was  made  before  a 
meeting  of  fruit  growers  and  fumigators  in  Los  Angeles  and  pro- 
voked deep  interest.  The  very  successful  work  against  the  purple 
vcnlc  at  Whittier  had  begun  to  be  evident  by  this  time  and  led  to 
the  prompt  adoption  of  the  system  by  the  orchardists  thereabout. 
Growers  from  other  localities,  inquiring  into  the  experience  with 
the  recent  innovation  at  Whittier,  commenced  to  sanction  its  adop- 
tion in  their  respective  districts,  so  that  by  the  end  of  the  funiiga- 


Fig.  2.  — Dosing  a  Tree.  This  View  Also  Shows  the  Scheduler  Securing  the 
Measurements  of  the  Next  Tree  in  the  Row.  'Original.* 


FUMIGATION  OF  CITRUS  TREES. 


39 


tion  season  of  190S  fully  a  dozen  outfits  in  various  parts  of  Los 
Angeles  and  Orange  Counties  were  using  the  new  method  in  prefer- 
ence to  the  old.  The  experience  of  the  first  season  lias  led  to  the 
rapid  and  successful  introduction  of  the  new  system  quite  generally, 
so  t hiit  it  now  has  heen  adopted  by  many  outfits  in  Los  Angeles, 
Orange,  Ventura,  and  Kiverside  Counties,  while  in  San  Bernardino 
County  it  is  used  almost  exclusively. 

Naturally  there  was  considerable  opposition,  at  the  commence- 
ment of  this  investigation,  on  the  part  of  the  professional  lumigators. 
Their  prejudice  has  been  overcome  to  a  large  extent  by  demonstra- 
tions and  personal  Cooperation,  and  many  of  them  are  now  endorsing 
the  new  methods.  The  chief  means  of  exploitation  have  been  lec- 
tures, demo nst  rat  ions,  and  personal  contact  with  the  fruit  growers, 
in  this  educational  campaign  the  assistance  of  many  county  horticul- 
tural officers  and  managers  of  citrus  associations  has  made  success 
far  easier  than  it  otherwise  would  have  been. 

The  rapid  and  general  adoption  of  the  new  method  indicates  its 
practical  economy,  for  new  ideas  are  not  adopted  by  California 
horticulturists  merely  for  the  sake  of  novelty.  The  primary  ques- 
tion before  the  grower  is  whether  in  the  long  run  the  new  system  of 
fim-.igation  is  more  economical  than  the  old  one.  The  new  system 
iias  been  used  in  and  about  'Whit  tier  for  nearly  two  years.  Having 
been  located  in  that  region,  the  writer  has  been  able  to  keep  in  touch 
with  the  condition  of  fumigation  thereabouts. 

A  year  ago  almost  all  fumigation  in  the  Whit  tier  and  Rivera  dis- 
tricts was  carried  on  under  the  new  method.  Packinghouse  st at is- 
tics  of  last  year's  crop  at  the  Whittier  and  Rivera  Citrus  Associations, 
which  handle  most  of  the  fruit  from  this  section  of  several  thousand 
acres,  showed  that  a  considerably  smaller  percentage  of  fruit  was 
discarded  because  of  being  infested  with  scale  than  during  any 
season  when  the  old  nun  hod  of  fumigation  was  practiced.  Such 
statistics  are  conclusive  and  their  significance  is  plain. 

One  of  the  writer's  early  contentions  was  that,  after  one  or  two  suc- 
cessive thorough  treatments  under  the  new  method,  using  the  proper 
dosage,  most  orchards  would  be  in  such  a  clean  condition  that  they 
could  go  without  treatment  at  least  every  other  year.  Indicating 
the  correctness  of  this  belief,  Mr.  William  Wood,  the  very  efficient 
former  horticultural  officer  for  the  Whittier  district,  states  that 
many  more  orchards  in  his  district  which  were  treated  during  1908 
under  the  new  svstem  were  sufficientlv  clean  not  to  need  fumigation 
the  following  season  than  has  been  the  case  under  the  old  method 
at  any  time  within  his  experience.  To  show  the  general  attitude  of 
the  growers  it  is  only  necessary  to  say  that  they  are  so  satisfied  as 
to  refuse  to  have  their  orchards  treated  except  under  the  new  pro- 
cedure. This  condition  at  Whittier  is  illustrative  of  what  is  taking 
67330°— Bull.  1)0— 12  4 


40  HYDROCYAXIC-ACID  GAS  FUMIGATION   IX  CALIFORNIA. 


place  in  many  other  districts  in  southern  California.  In  short,  the 
experience  with  this  new  method  of  fumigation  has  been  so  success- 
ful throughout  the  southern  fruit-growing  sections  that  it  is  only  a 
matter  of  time  when  it  necessarily  must  entirely  supplant  the  old 
methods. 

THE  CHEMICALS  IN  FUMIGATION. 

For  the  generation  of  hydrocyanic-acid  gas  in  fumigation  potas- 
sium cyanid,1  sulphuric  acid,  and  water  are  necessary.  The  hydro- 
cyanic-acid gas  is  produced  by  the  action  of  the  sulphuric  acid  on 
the  cyanid  of  potassium.  Under  the  early  methods  of  generating 
hydrocyanic-acid  gas  the  cyanid  was  dissolved  in  water  before  being 
used.  At  the  present  time  cyanid  is  used  entirely  in  the  crystal 
form.  The  water  is  first  measured  and  poured  into  the  generating 
vessel.  The  required  amount  of  acid  is  then  added  to  the  water, 
producing  a  great  increase  of  the  temperature  of  the  mixture.  While 
the  mixture  is  hot  it  should  be  placed  beneath  the  tree  and  the 
cyanid  added.  If  permitted  to  cool  before  the  cyanid  is  added, 
the  generation  of  gas  will  not  only  be  slower  than  with  the  heated 
mixture,  but  the  amount  of  available  gas  will  be  decreased,  thus 
making  the  operation  more  expensive,  and  necessarily  less  efficient. 

POTASSIUM  CYANID  (KCX). 

An  imported  potassium  cyanid  designated  as  98  to  99  per  cent 
pure  is  used  almost  exclusively  for  fumigation  purposes  in  southern 
California.  This  imported  potassium  cyanid  has  been  employed 
throughout  the  held  investigations  of  this  bureau  and  is  referred  to 
in  this  bulletin  whenever  cyanid  is  mentioned,  unless  specific  men- 
tion of  another  grade  is  given.  Analyses  of  seven  samples  taken 
promiscuously  in  the  field  during  this  investigation  averaged  98.1 
per  cent  pure,  the  poorest  sample  being  97.28  per  cent.  The  gen- 
eral fumigation  experience  with  such  a  high  grade  of  potassium 
cyanid  has  been  very  satisfactory.  This  cyanid  is  purchased  in 
cases  containing  approximately  200  pounds. 

The  potassium  cyanid  most  frequently  used  in  California  for 
fumigation  purposes  is  commonly  spoken  of  as  " German"  cyanid,  it 
being  generally  known  that,  this  chemical  is  imported' from  Ger- 
many. This  term  "German"  has  been  used  in  distinguishing  the 
regular  potassium  cyanid  from  another  kind  popularly  known  as 
■  American"  cyanid,  which  has  had  a  very  limited  and  unsatisfactory 
usage  for  a  number  of  years.  This  "American."  cyanid  was  known 
to  be  of  a  generally  much  harder  composition  and  slower  in  gener- 
ation. Because  this  latter  cyanid  is  made  in  America  the  general 
belief  has  prevailed  among  fumigatora  that  it  is  impossible  to  manu- 


1  For  the  use  of  sodium  cyanid,  see  Part  it  of  this  Bulletin. 


FUMIGATION  OF  CITRUS  TREE* 


41 


facture  as  suitable  a  grade  of  fumigating  cyanid  in  this  country  as  in 
( in  many.  Such  a  belief  is,  of  course,  erroneous.  Chemical  analysis 
of  this  so-called  "American"  cyanid  has  shown  it  to  he  not  potas- 
sium cyanid  hut  sodium  cyanid.  which  is  a  very  different  article. 
No  potassium  cyanid  is  manufactured  in  this  country. 

A  potassium  cyanid  1  guaranteed  toheMS-00  per  cent  pure  should 
be  used,  as  experience  gained  during  this  investigation,  as  well  as 
that  from  commercial  operations,  has  proved  this  grade  of  cyanid 
to  be  uniformly  successful.  Moreover,  such  a  high-grade  article  is 
quite  free  of  sodium  chlorid  (common  salt),  the  detrimental  action 
of  which  is  explained  later. 

Cyanid  should  he  exposed  to  damp  air  as  little  as  possible,  as  it  is 
decomposed  by  moisture.  Analysis  of  a  sample  exposed  to  the  air 
for  a  few  months  showed  it  to  he  several  per  cent  less  pure  than 
originally.  Such  a  cyanid,  or  even  one  which  has  become  moistened 
by  only  a  few  days1  exposure  to  the  weather,  i^  Blower  in  generating 
its  gas,  and  this  is  an  objectionable  feature  in  fumigation.  After 
opening  in  the  field,  the  case  of  cyanid  Bhould  be  protected  by  a  tight 
cover  which  will  ward  off  the  action  of  dew  or  rain. 

BumUBIG   \<  id  (n.so,). 

A  commercial  sulphuric  acid  (II..SOJ,  m°  Baunie,  which  is  approxi- 
mately !):!  per  cent  pure,  should  be  used*  Sulphur  is  the  basic 
ingredient  in  the  manufacture  of  sulphuric  acid.  Generally  speaking, 
an  acid  in  which  the  sulphur  is  obtained  from  brimstone  is  preferable 
to  one  made  from  iron  pyrites.  The  reason  18  that  those  which  are 
made  from  a  brimstone  base  usually  contain  fewer  impurities  than 
those  made  from  iron  pyrites.  If  the  impurities  be  eliminated, 
however,  the  sulphuric  acid  made  from  the  one  is  as  satisfactory  in 
fumigation  ;.s  that  made  from  the  other. 

The  commonest  impurity  in  sulphuric  acid  is  sulphate  of  iron 
(FeSO,).  This  often  occurs  in  acids  made  from  pyrites,  and  some- 
times to  a  very  great  extent.  It  add-  a  milky  appearance  to  the 
acid.  The  action  of  acid  on  long-used  iron  drums  also  causes  the 
formation  of  sulphate  of  iron,  evidenced  by  the  whitish  appearance 
of  the  -'settlings"  or  the  acid  at  the  bottom  of  the  drum.  The 
Writer  has  used  acid  containing  considerable  sulphate  of  iron  without 
any  apparent  injury  to  citrus  trees  or  fruit.    Nevertheless  acid  con- 

1  Experimentation  during  this  investigation  has  shown  that  a  high  grade  of  sodium  cyanid  will  produce 
exactly  as  satisfactory  results  as  a  high-grade  potassium  cyanid.  No  sodium  cyanid  less  than  12ft- 130  per 
cent  pure  (as  reckoned  in  terms  of  a  potassium  cyanid)  should  be  used.  If  a  suitable  potassium  cyanid  is 
not  available  then  purchase  a  sodium  cyanid  of  the  purity  mentioned.  A  pound  of  this  sodium  cyanid 
contains  approximately  one-fourth  more  available  gas  than  a  pound  of  potassium  cyanid.  Hence,  if  used, 
the  dosages  employed  should  l>e  one-fourth  less  than  those  mentioned  in  this  bulletin.  The  proportion  ol 
chemicals  is  also  different.  A  1-1 J-2  formula  is  advised:  that  is.  to  each  ounce  (average)  of  126-130  per 
oenl  90dimfl  cyanid  use  1J  ounces  (liquid  measure)  of  sulphuric  acid  and  2  ounces  of  water. 

For  a  thorough  treatment  of  sodium  cyanid  in  relation  to  fumigation  see  Part  II  of  this  Bulletin. 


42  HYDROCYAXIC-ACID  GAS  FUMIGATIOX  IX  CALIFORXIA. 


taining  large  quantities  of  sulphate  of  iron  should  be  avoided  for 
the  same  reasons  that  cyanid  containing  large  quantities  of  impu- 
rities should  be  avoided,  even  though  the  impurities  are  apparently 
harmless. 

Traces  of  nitric  acid  (HN03)  are  sometimes  present  in  sulphuric 
acid.  For  several  years  an  opinion  has  been  current  in  California 
that  nitric  acid  when  present  in  sulphuric  acid  used  in  fumigation 
would  result  in  the  burning  of  fruit.  Burning  of  fruit  has  occurred 
to  a  greater  or  less  extent  throughout  the  history  of  fumigation, 
yet  in  recent  years,  because  this  damage  has  sometimes  taken  place 
when  an  acid  made  from  pyrites  was  being  used,  and  in  which  a 
trace  of  nitric  acid  was  sometimes  present,  the  belief  has  become 
quite  general  among  fumigators  that  such  an  acid  was  unsafe. 
The  theory  proposed  as  the  cause  was  that  the  heat  produced  in 
generating  the  hydrocyanic-acid  gas  drove  off  the  nitric  acid  in  the 
form  of  a  vapor,  which,  coming  in  contact  with  the  cooler  surface  of 
the  fruit,  condensed,  resulting  in  a  burn  or  pit.  Careful  experiments 
were  recently  carried  out  in  order  to  decide  this  point.  Eight  orange 
and  lemon  trees  well  laden  with  fruit  were  treated  on  three  different 
nights,  using  sulphuric  acid  containing  from  1  to  10  per  cent  of  pure 
nitric  acid.  Dosage  schedule  No.  1  was  followed.  The  exposure 
was  one  hour.  No  pitting  or  burning  resulted  with  any  of  the 
strengths  used.  As  these  amounts  of  nitric  acid  are  far  in  excess  of 
the  quantities  ever  found  in  commercial  sulphuric  acid  it  can  be 
safely  concluded  that  there  is  no  danger  of  burning  as  a  result  of 
the  presence  of  nitric  acid  in  the  commercial  sulphuric  acid. 

Traces  of  arsenic,  lead,  or  zinc  are  sometimes  found  in  commercial 
sulphuric  acid,  yet  in  all  samples  of  acid  analyzed  during  this  inves- 
tigation the  quantity,  when  present,  has  been  so  small  as  to  demand 
no  consideration  as  a  source  of  injury. 

Sulphuric  acid  is  purchased  largely  in  iron  drums  containing  from 
1,500  to  2,000  pounds.  Glass  carboys  of  about  10  gallons'  capacity 
are  sometimes  used.  The  drums,  because  of  their  great  weight  ,  are 
seldom  taken  into  the  held.  A  common  and  convenient  method 
is  to  roll  the  drums  onto  an  elevated  platform  at  the  source  of  sup- 
plies. The  acid  is  then  removed  into  glass  carboys  or  some  other 
receptacle  for  carriage  into  the  field.  Two  or  three  carboys  usually 
will  contain  enough  acid  for  one  night's  run  of  an  outfit  of  tents. 

Care  should  he  observed  in  handling  this  acid."  Rubber  gloves 
arc  advisable.  If  some  acid  accidentally  reaches  the  flesh  hasten  to 
wash  t  he  affected  parts  with  water. 

THE  AMOUNT  OF  BULPHUBIO  ACID  NECESSARY. 

Chemical  combinations  take  place  with  definiteness  under  the 
same  conditions;  thai  is,  given  the  same  conditions,  when  one 
chemical  acts  upon  another  in  the  production  of  a  third  substance, 


FUMIGATION  OF  CITRUS  TREES. 


43 


the  proportion  between  the  first  two  chemicals  is  practically  the 
same.  Such  is  the  case  when  sulphuric  acid  acts  upon  potassium 
cyanid  in  producing  hydrocyanic-acid  gas.  A  given  amount  of 
cyanid  requires  a  given  a  mount  of  sulphuric  acid  of  a  fixed  degree 
of  purity  in  order  to  thoroughly  utilize  the  quantity  of  cyanid 
employed,  evolving  the  maximum  amount  of  gas,  and  carrying  the 
reaction  to  completion.  A  quotation  from  a  letter  received  from 
Dr.  J.  K.  Haywood,  of  the  Bureau  of  Chemistry,  of  this  department, 
illiist  rates  this  point : 

In  the  action  of  sulphuric  acid  on  potassium  cyanid  approximately  four-fifths  of 
an  r>unc«'  (avoirdupois)  of  (J3  per  cent  acid  is  used  up  for  every  ounce  of  98  per  cent 
cyanid.1 

Expressed  in  fluid  ounces  four-fifths  of  an  ounce  avoirdupois  equals  about  0.42  of 
a  fluid  ounce.  We  may  say  that  theoretically  1  ounce  avoirdupois  of  98  per  cent 
potassium  cyanid  needs  0 .12  of  a  fluid  ounce  of  ordinary  commercial  sulphuric  acid 
(98  per  cent  i  to  convert  it  entirely  to  hydrocyanic  acid.  Since  it  is  always  best  to 
have  -ome  excess  of  the  acid  to  carry  the  reaction  to  completion,  it  is  probable  that 
three -fourths  of  a  fluid  ounce  of  commercial  sulphuric  acid  is  ample  in  practice  to 
convert  1  ounce  avoirdupois  of  98  per  cent  potassium  cyanid  to  hydrocyanic  acid. 
It  1  fluid  ounce  of  the  commercial  sulphuric  acid  is  used  it  will  certainly  leave  a 
considerable  excess  of  sulphuric  acid.  It  IB  perfectly  possible,  however,  that  this 
SXC68e  of  sulphuric  acid  is  of  value  in  heating  up  the  mixture  so  that  more  of  the 
hydrocyanic  acid  is  liberated  and  not  absorbed  by  the  liquid. 

Two  series  of  held  experiments  were  performed  which  were  identical 
in  all  respects  except  that  in  the  first  1  ounce  of  sulphuric  acid  was 
used  while  in  the  other  1  \  ounces  were  used  to  each  ounce  of  potas- 
sium cyanid.  Analysis  of  the  residue  by  the  Bureau  of  Chemistry, 
of  this  department ,  showed  that  the  react  inn  was  as  perfect  with  the 
Smaller  proportion  of  acid  as  with  the  Ukrgpr.  The  addition  of  a 
great  excess  of  acid  might  even  result  in  an  impediment  to  rapid  work 
in  the  field.  As  an  explanation  of  this  condition  it  might  be  stated 
that  the  residue  always  contains  a  substance  which  is  soluble  in 
water  alone,  but  the  presence  of  a  large  excess  of  sulphuric  acid  will 
cause  it  to  crystallize  and  solidify.  This  latter  condition  will  fre- 
quently occur  if  more  than  equal  parts  of  acid  to  cyanid  are  used, 
especially  so  with  the  smaller  dosages.  The  removal  of  solidified 
residue  necessitates  loss  of  time. 

Summing  up,  it  may  he  said  that  1  fluid  ounce  of  commercial 
sulphuric  acid  (!)3  per  cent)  to  1  ounce  (avoirdupois)  of  98  per  cent 
potassium  cyanid  is  certainly  enough  to  carry  the  reaction  to  com- 
pletion in  the  liberation  of  hydrocyanic-acid  gas  and  is  perhaps  an 
unnecessarily  large  amount.  In  practical  field  work,  where  dosages 
of  varying  sizes  are  constantly  being  used,  it  is  very  convenient  to 
reckon  the  acid  in  the  same  number  of  parts  as  the  cyanid.  The 
use  of  1  part  (fluid  measure)  of  acid  to  each  part  (1  ounce  avoirdu- 
pois) of  cyanid  is  therefore  recommended. 


s  The  reaction  is  M  follows:  2  KCN+  ll2SO«=K2SOH-2  HCN. 


44  HYDROCYANIC- ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


WATER  AS  A  FACTOR  IN  FUMIGATION. 

There  are  several  reasons  why  water  should  always  be  employed 
in  fumigation.  It  is  very  useful  in  dissolving  the  potassium  cyanid 
and  hastening  and  completing  the  chemical  reaction  with  the  acid. 
A  piece  of  cyanid  thrown  into  a  mixture  of  acid  and  water  immediately 
gives  up  a  portion  of  its  mass  in  solution.  Scarcely  has  the  cyanid 
dissolved  when  it  is  partially  converted  into  gas.  The  heat  liberated 
during  this  process  assists  in  forcing  the  solution  of  more  cyanid 
which  is  also  partially  converted  into  gas.  This  continues  until 
the  chemicals  are  exhausted  and  the  reaction  ceases. 

Potassium  sulphate,  a  solid,  is  the  by-product  resulting  from  the 
reaction  by  which  hydrocyanic-acid  gas  is  produced.  Water  dissolves 
the  potassium  sulphate  as  it  forms  and  prevents  it  from  coating  the 
cyanid  not  yet  in  solution.  In  the  presence  of  an  insufficient  amount 
of  water,  the  potassium  sulphate  is  not  completely  dissolved,  but 
forms  a  coating  on  the  pieces  of  cyanid,  preventing  the  sulphuric 
acid  from  penetrating  to  it,  and  thereby  retarding,  or  even  in  part 
preventing,  the  reaction.  In  such  cases  this  undissolved  potassium 
sulphate  usually  solidifies,  causing  the  pots  to  " freeze."  This 
phenomenon  always  occurs  where  the  formula  is  1-1-1,  or  where 
the  same  amounts  of  water,  acid,  and  cyanid  are  used.  On  agitating 
the  residue  by  stirring,  it  is  almost  always  possible  to  find  small 
pieces  of  undissolved  cyanid  enveloped  in  a  coating  of  the  potassium 
sulphate.  Ordinarily,  when  the  residue  is  stirred  the  particles  of 
cyanid  are  removed,  to  some  extent,  from  this  envelope  of  potassium 
sulphate,  allowing  some  of  the  unused  acid  to  reach  them,  and  thus 
evolving  a  small  amount  of  gas  without  the  addition  of  more  acid. 
Under  these  conditions,  however,  the  reaction  is  never  complete, 
and  it  is  highly  desirable,  therefore,  to  add  sufficient  water  at  the 
beginning  to  dissolve  all  the  potassium  sulphate. 

Recalling  the  statements  made  in  discussing  the  amount  of  sul- 
phuric acid  to  use,  it  is  seen  that  the  " congealing"  or  "freezing" 
of  the  residue  in  the  generating  jars  is  due  to  either  or  both  of  two 
conditions:  (1)  An  insufficient  amount  of  water  to  completely  dis- 
solve the  sulphate  of  potassium,  or  (2)  a  large  excess  of  sulphuric 
acid,  whereby  the  water  is  rendered  less  capable  of  taking  into 
solution  the  same  amount  of  sulphate  as  it  otherwise  would. 

Another  very  important  function  of  the  water  in  the  reaction  is 
the  heal  produced  by  the  union  of  the  sulphuric  acid  and  water. 
Potassium  cyanid  introduced  into  this  heated  mixture  gives  off 
hydrocyanic-acid  gas  much  more  quickly  and  thoroughly  than  at  a 
lower  temperature,  and  in  field  work  rapid  generation  of  gas  is 
essential, 

The  art  ion  of  pure  or  highly  concentrated  sulphuric  acid  on  potas- 
sium cyanid  results  in  a  very  different  chemical  reaction  than  when 


FUMIGATION  OF  CITRUS  TREES. 


45 


the  acid  lias  been  diluted  with  water.  With  very  dilute  sulphuric 
acid  and  up  to  a  strength  of  1  part  acid  to  1  part  of  water,  which  is 
aa  concentrated  a  mixture  as  is  ever  used  in  fumigation  work,  nearly 
pure  livdmcyauic-acid  gas  is  given  ofi".  By  decreasing  the  propor- 
tion of  water  used  below  1  part,  the  amount  of  hydrocyanic-acid 
gas  resulting  is  also  decreased  until,  when  concentrated  sulphuric 
acid  act -on  a  cyanid,  hydrocyanic-acid  gas  La  not  given  off,  but  rather 
an  entirely  dhTerent  gas  called  carbon  monoxid.1 

THK   EFFECT  OF  DIFFERENT  PROPORTIONS    OF  WATER. 

On  U  mperaturt  of  gas. — Anyone  who  has  watched  the  escaping  gas 
and  steam  from  the  reaction  of  potassium  cyanid  and  sulphuric  acid 
wherein  different  proportions  of  water  were  used  could  not  fail  to 
notice  that  the  violence  with  which  the  generation  starts  and  the 
gas  is  given  oiF  is  apparently  greatest  with  the  smaller  proportions  of 
water.    Experiments  carried  on  by  this  investigation  showed  that 


PFfOPORT/OHS  OF 
CYAN/&  ACID  WATER 


PER  CENT  OF  GAS  G/VEA/  OFF 
10%    20%    30      40%    50%    60%    70%    80%    90%  100% 


Fig.  10.— ("hart  showing  total  amount  of  gas  evolved  when  different  proportions  of  water  are  used. 

( Author's  illustration.) 

the  temperature  of  the  escaping  gas  was  considerably  higher  with 
smaller  proportions  of  water  than  with  the  larger  proportions.  In  one 
experiment  the  highest  temperature  of  the  escaping  gas  was  124°  F. 
w  ith  1  pari  of  w  ater,  but  only  90°  F.  with  8  parts.  The  temperature 
w  as  approximately  uniform  with  from  1  to  4  parts  of  water. 

On  amount  of  available  pas. — The  Bureau  of  Chemistry  of  this 
department,  at  the  request  of  the  Bureau  of  Entomology }  performed 
an  experiment  to  determine  the  amount  of  hydrocyanic-acid  gas 
available  when  generated  with  proportions  of  water  varying  from  l 
to  8  parts.  The  results  have  been  incorporated  in  the  accompanying 
chart  (fig.  10). 

1  n  these  experiments  commercial  sulphuric  acid  66°  Ban  me,  analyz- 
ing 92.77  per  cent  pure,  and  potassium  cyanid  97.12  per  cent  pure 
were  used.    Three  ounces  (fluid)  of  sulphuric  acid  and  3  ounces 


i  See  Pari  III  of  this  Bulletin. 


46  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

(avoirdupois)  of  potassium  cyanid  were  employed  in  each  experi- 
ment, and  3,  6,  9,  12,  15,  18,  21,  and  24  ounces,  respectively,  of 
water  were  used  in  the  different  experiments. 

From  this  chart  it  is  evident  that  with  the  grades  of  acid  and 
cyanid  mentioned  the  largest  amount  of  gas  is  available  from  2  parts 
of  water.  As  the  proportion  of  water  is  increased  above  2  parts  the 
available  gas  is  decreased,  until  with  8  parts  of  water  we  obtain  only 
about  43  per  cent  of  gas,  or  less  than  one-half  as  much  as  with  2 
parts.  In  other  words,  1  ounce  of  cyanid  and  1  ounce  of  acid  in 
combination  with  2  ounces  of  water  will  produce  much  more  avail- 
able gas  than  2  ounces  of  cyanid  and  2  ounces  of  acid  with  16  ounces 
of  water. 

The  cause  for  the  smaller  amount  of  gas  with  1  part  of  water  has 
been  explained  on  page  44.  One  of  the  principal  reasons  for  the 
decrease  of  the  amount  of  gas  as  we  go  above  2  parts  of  water  is  that 
the  temperature  of  the  acid-water  mixture  decreases  as  the  pro- 
portion of  water  increases.  With  dosages  of  5  ounces  of  cyanid  the 
temperature  was  found  to  be  190°  F.  where  two  parts- of  water  were 
used,  but  only  125°  F.  with  8  parts  of  water.  The  hotter  the  acid- 
water  mixture  the  quicker  and  more  violent  the  reaction  with  the 
cyanid  will  be.  Secondly,  hydrocyanic-acid  gas  is  very  soluble  in 
water.  As  the  cyanid  is  immersed  during  the  reaction,  the  gas  has 
to  rise  through  the  liquid  in  order  to  escape.  Less  gas  will  .be 
absorbed  by  rapid  evolution  through  a  small  amount  of  water  than 
by  slower  rise  through  a  large  amount. 

The  proportion  of  water  used  by  different  fumigators  under  the  old 
system  has  varied  all  the  way  from  2  to  8  parts,  some  men  even 
varying  widely  in  their  individual  work.  In  brief,  the  method  fol- 
lowed by  the  "generator"  man  in  dosing  has  been  that  on  coming  to 
a  tree  he  first  looks  at  his  can  of  cyanid  for  that  tree  and  then  makes 
a  guess  as  to  how  many  ounces  it  contains.  If  he  is  using  2  parts  of 
water  he  will  use  twice  the  amount  that  he  thinks  there  is  cyanid  in 
the  can;  if  8  parts,  then  8  times  the  amount  of  cyanid  he  thinks 
there  is  in  the  can.  A  very  few  outfits  have  measured  the  water  in 
graduated  beakers;  the  majority  of  receptacles  used  have  varied  all 
the  way  from  half-pint  dippers  to  quart  dippers,  quart  pitchers,  or 
even  old  tin  cans.  Think  of  measuring  with  accuracy  the  amount  of 
water  for  a  tree  requiring  4  ounces  of  cyanid  with  a  quart  pitcher! 
The  writer  has  frequently  seen  fumigators,  in  measuring  the  water 
Tor  a  tree,  first  measure  out  what  they  thought  to  be  the  proper 
amount,  then  hesitate  as  to  whether  it  was  enough,  and  finally  dip 
out  a  second  or  even  a  third  portion.  Those  second  and  third  dips 
meant  less  available  gas,  and  the  common  multifold  guessing  in  the 
meamirp  of  water  under  the  old  system  has  been  directly  responsible 
for  irregular  results. 


FUMIGATION  OF  C  ITRUS  TREES 


4  7 


It  has  boon  a  common  practice  among  rumigators  to  increase  the 
dosage  when  fumigating  a  tree  severely  infested  with  scale.  It  also 
has  hern  a  common  practice — in  fact  so  common  as  to  be  almost 
universal — to  increase  the  proportion  of  water  when  using  such  heavy 
dosages.  It  was  believed  that  this  extra  water  reduced  the  tempera- 
ture of  the  gas,  thereby  preventing  the  burning  of  the  foliage.  Very 
naturally,  the  use  of  extra  water  might  produce  less  injury,  but  tins 
would  not  be  due  to  the  reduction  of  temperature,  as  has  been 
believed,  but  to  the  decrease  of  the  amount  of  gas  given  oil*.  This 
practice  has  caused  a  great  waste  of  cyanid  and  wide  disparity  in 
results.  Indeed,  the  writer  believes  that  no  one  factor  has  had  more 
to  do  with  the  wide  variation  in  results  secured  in  fumigating  citrus 
trees  than  has  tins  erratic  use  of  water. 

THE  CORRECT  PROPORTION  OF  WATER. 

It  has  been  shown  that  2  parts  of  water  to  1  pari  each  of  cyanid 
and  sulphuric  acid  will  produce  the  maximum  amount  of  available  gas. 
It  is  impractical,  however,  to  use  2  parts  of  water  in  held  work 
because  with  this  proportion  the  residue,  especially  when  small 
dosages  are  used,  will  frequently  solidify  within  one  hour's  time, 
which  is  the  usual  period  for  leaving  tents  on  the  trees.  Although 
this  proportion  of  water  apparently  is  sufficient  to  dissolve  the  sul- 
phate at  first  so  that  a  complete  reaction  takes  place,  it  appeals 
Unable  to  hold  the  sulphate  in  solution  1  < ►  1 1  —  enough  afterwards  to 
prevent  "freezing."  This  phenomenon  is  an  impediment  lo  rapid 
field  work,  for  some  little  time  is  required  to  remove  this  congealed 

residue  from  the  constricted-neck  generating  pots  in  common  use. 
It  is  evident  in  this  instance  that  a  "frozen"  generator  does  not 
imply  an  incomplete  generation,  although  in  some  other  cases  the  resi- 
due left  may  be  congealed  and  the  generation  incomplete.  With  3 
parts  of  water  the  residue  seldom  congeals  and  this  is  the  proportion 
recommended  by  and  used  in  all  the  held  work  of  the  writer.  With 
dosages  of  12  ounces  of  cyanid  or  above,  a  2l-ounce  ratio  of  water  can 
be  used  without  danger  of  "freezing."  The  water  should  be  measured 
carefully  with  a  glass  or  dipper  graduated  to  ounces. 

THE  MOST  ECONOMICAL  PROPORTION  OF  CHEMICALS  TO  TSE. 

In  the  preceding  discussion  it  has  been  shown  that  for  various 
reasons  1  fluid  ounce  of  commercial  sulphuric  acid  and  1  ounce 
(avoirdupois)  of  06  to  100  per  cent  potassium  cyanid  in  combination 
with  3  fluid  ounces  of  water  give  a  complete  reaction.  Thus  the 
1—1—3  formula,  hitherto  recommended  by  the  Bureau  of  Entomology, 
is  fully  indorsed  for  fumigation  work  in  the  field. 

A  review  of  the  use  of  hydrocyanic-acid  gas  for  fumigation,  both  in 
California  and  elsewhere,  shows  frequent  divergence  from  the  more 


48  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

economical  and  satisfactory  proportion  of  chemicals  indicated  above. 
However,  since  the  results  of  this  investigation  have  been  given  out 
the  former  erratic  methods  of  measuring  water  have  almost  entirely 
disappeared.  The  usual  practice  now  is  to  use  3  parts  of  water 
which  is  generally  measured  in. graduated  receptacles.  The  systema- 
tizing of  the  use  of  water  has  been  one  of  the  greatest  accomplish- 
ments of  the  present  investigation. 

In  such  special  treatments  as  that  of  nursery  stock,  mills,  houses, 
and  the  like,  where  the  extra  time  required  to  remove  the  congealed 
residue  would  in  no  way  interfere  with  the  rapid  and  economical 
progress  of  the  work,  2  parts  (ounces)  of  water  to  each  part  (ounce) 
of  cyanid  is  recommended. 

THE  AMOUNT  OF  CHEMICALS  IN  VERY  SMALL  DOSAGES. 

The  results  in  the  fumigation  of  small  trees  requiring  from  1  to  3  or 
4  ounces  of  cyanid  have  generally  been  much  less  satisfactory  than  for 
the  larger  sizes.  If  the  amounts  of  chemicals  used  for  such  small 
dosages  in  large  generating  pots  are  always  in  proportion  to  the  1-1-3 
formula  the  reaction  will  sometimes  be  slow  and  incomplete.  This  is 
especially  the  case  if  pieces  of  cyanid  of  such  size  as  to  project  above 
the  surface  of  trie  liquid  are  used.  In  order  that  the  cyanid  may  be 
entirely  covered  by  the  liquid  the  entire  dosage  should  be  not  in  one 
piece  but  preferably  in  two  or  more  smaller  pieces.  It  is  also  advis- 
able to  increase  the  amount  of  the  acid-water  mixture  to  a  slight 
extent  in  such  cases.  An  extra  ounce  of  acid  and  3  extra  ounces 
of  water  will  usually  suffice. 

MIXING  THE  CHEMICALS. 

It  is  preferable  to  pour  the  water  into  the  generator  first  and  then 
add  the  acid.  The  pouring  of  the  water  onto  the  acid  is  more  likely 
to  cause  splashing  of  the  acid  from  the  jar  onto  the  fumigator.  When 
the  acid  and  water  are  in  readiness  for  generating  the  gas  the  fumi- 
gator adds  the  pieces  of  cyanid  to  the  mixture  and  hastily  retreats. 
As  already  stated,  the  cyanid  should  be  added  while  the  mixture  of 
water  and  acid  is  hot.  Other  investigators1  have  called  attention 
to  this,  while  experiments  performed  by  the  Bureau  of  Chemistry  of 
tins  department  show  that  the  reaction  with  a  cool  solution  is  very 
inferior  to  one  when  the  heat  is  great.  Potassium  cyanid  added 
1<>  the  mixture  of  acid  and  water  when  hot  lost  10.68  per  ceni  of 
hydrocyanic-acid  gas,  while  the  same  cyanid  added  to  a  mixture  of 
acid  and  water  while  cold  lost  23.25  per  cent,  a  difference  of  more 
than  12  per  cent.  The  cyanid  should  never  be  placed  in  the  water 
before  the  acid  Is  added.  If  the  acid  is  added  to  the  cyanid  in  solu- 
tion, ;i  very  violent  reaction  takes  place,  which  will  sometimes  throw 


•  Gossanl,  Bill.  (17,  Fla.  Agr.  Exp.  Sfa. 


FUMIGATION   OF   (TTHUS    THE  ICS. 


49 


much  of  the  liquid  from  the  vessel.  In  one  instance  about  1  pound 
of  cyanid  was  dissolved  in  water  in  a  2-gallon  generator.  Acid  was 
then  added,  producing  a  disturbance  so  violent  as  to  throw  some  of 
the  liquid  almost  to  the  top  of  a  2-story  barn. 

The  cyanid  should  be  in  pieces  anywhere  from  the  size  of  an  English 
walnut  to  that  of  a  good-sized  lemon.  The  smaller  pieces  should  be 
iimmI  iii  the  small  dosages.  Powdered  cyanid  should  be  avoided  in  BO 
far  as  possible.  Where  purchased  in  large  boxes  there  is  always  a 
considerable  quantity  of  fine  material  at  the  bottom.  Entire  dosages 
for  a  tree  should  never  be  composed  entirely  of  this  character  of 
cyanid  or  a  violent  reaction  will  take  place,  blowing  much  of  the 
fine  particles  out  of  the  generator  and  endangering  the  tent  as  well 
as  the  operator.  This  fine  cyanid  is  most  economically  and  satis- 
factorily disposed  of  by  using  it  in  small  quantities  along  with  lumps. 

The  generation  of  gas  has  practically  ceased  at  the  expiration  of 
from  three  to  five  minutes. 

Many  writers  OB  fumigation  recommend  the  use  of  paper  bags  for 
holding  each  dosage  when  placet!  in  the  generating  pot.  These  bags 
are  used  largely  to  retard  the  reaction  so  that  the  operator  may 
retreat  to  BOme  distance  before  the  generation  commences  or  else 
to  prevent  slopping.  The  writer's  own  experience,  as  well  as  some 
principles  previously  mentioned  in  this  chemical  discussion,  would 
lead  to  advising  against  t lie  use  of  paper  bags.  The  retardation  of 
generation  is  so  marked  in  the  case  of  small  dosages  in  heavy  paper 
bags  that  the  amount  of  gas  resulting  must  be  considerably  less 
than  if  the  cyanid  had  been  introduced  in  a  free  state.1  By  the  exer- 
cising of  a  slight  amount  of  care  in  introducing  cyanid  in  the  free 
state  into  a  generating  vessel  there  is  no  danger  of  the  operator 
being  affected  by  the  gas  or  of  the  acid  being  slopped  out.  Neither 
will  the  generating  pots  boil  over  if  the  amounts  scheduled  on  page 
24  are  used.  Fine  or  powdered  cyanid  should  never  be  used  in 
houses.  In  household  work  sheets  of  heavy  paper  should  be  placed 
underneath  the  generators. 

EFFECT  OF  THE   PRESENCE  OF  SODIUM   OHLOBID  OX  THE  AMOUNT  OF 

GAS  GIVEN  OFF. 

Practically  all  commercial  cyanid  contains  more  or  less  common 
salt,  technically  known  as  sodium  chiorid.  The  action  of  this  salt' 
in  connection  with  fumigation  demands  consideration.  It  has  been 
found,  when  sodium  chiorid  is  present  in  the  reaction  of  sulphuric 
acid  on  a  cyanid  in  the  production  of  hydrocyanic-acid  gas,  that 
this  chiorid  salt  produces  a  secondary  reaction  which  liberates  an 
acid  called  hydrochloric  acid,  and  that  this  liberated  hydrochloric  acid 


'  If  paper  sacks  arc  employed,  they  should  he  of  thin  paper,  or  slit  to  allow  the  free  action  of  the  acid  on 
the  cyanid.— C.  L.  M. 


50  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

immediately  attacks  the  hydrocyanic-acid  gas  and  decomposes  it  to  a 
great  extent.  Hence,  as  the  presence  of  sodium  chlorid  in  a  cyanid 
produces  a  partial  decomposition  of  the  hydrocyanic-acid  gas  when 
liberated,  the  ultimate  result  is  that  less  gas  is  given  off  than  from 
a  cyanid  of  the  same  degree  of  strength  which  is  free  of  it.1  Exten- 
sive experiments  carried  out  by  the  Bureau  of  Chemistry  of  this 
department  showed  that  the  presence  of  sodium  chlorid  in  a  reaction 
causes  a  very  marked  decomposition  of  the  hydrocyanic  acid. 
Experiments  with  two  different  cyanids  each  of  which  has  had  a 
limited  usage  in  California  showed  that  the  amount  of  sodium 
chlorid  in  one  caused  a  decomposition  of  9.76  per  cent  of  the  total 
hydrocyanic  acid,  the  other  of  34.07  per  cent.  An  experiment  per- 
formed with  a  cyanid  having  a  very  large  quantity  of  sodium  chlorid 
in  the  reaction  resulted  in  a  decomposition  of  over  92  per  cent  of  the 
total  amount  of  gas,  only  a  little  over  7  per  cent  being  evolved. 

The  results  of  these  experiments  bring  to  our  attention  a  second 
requirement  in  the  purchasing  of  a  cyanid.  That  it  be  of  a  certain 
degree  of  purity  is  no  longer  the  only  consideration.  It  is  of  equal 
importance  that  the  cyanid  be  practically  free  of  sodium  chlorid. 
Possibly  extensive  and  expensive  refining  would  be  necessary  to 
eliminate  all  traces  of  sodium  chlorid  from  a  cyanid.  Such  a  condi- 
tion would  be  preferable  but  can  not  be  demanded  at  the  risk  of 
increased  cost.  We  can,  however,  reasonably  expect  a  high  degree 
of  purity,  and  the  writer  would  condemn  as  unsuitable  for  use  in 
fumigation  any  cyanid  containing  in  excess  of  1  per  cent  of  sodium 
chlorid.  This  does  not  mean  that  every  cyanid  used  should  be 
examined  to  determine  if  it  contains  in  excess  of  this  quantity  of 
sodium  chlorid.  A  potassium  cyanid  98-99  per  cent  pure  has  such 
a  small  margin  for  impurities  that  it  will  not  contain  any  objection- 
able quantity.  A  potassium  cyanid  guaranteed  as  98-99  per  cent 
pure  can  be  used  with  entire  safety  provided  its  purity  measures  up 
With  the  guarantee. 

NATURE   OF  THE  RESIDUE. 

The  residue  resulting  from  the  generation  of  hydrocyanic-acid  gas 
Is  usually  a  bluish  or  greenish  colored  liquid  consisting  for  the  most 
pari  of  water.  It  also  contains  sulphate  of  potassium,  more  or  less 
sulphuric  acid,  and  some  hydrocyanic  acid  held  in  solution.  This 
combination  of  substances  is  of  a  very  poisonous  nature.  Neverthe- 
less, some  writ eis  on  fumigation,  considering  the  plant-food  elements 
which  this  residue  contains,  have  advised  that  the  residue  was  of 
much  importance  as  a  fertilizer  and  should  be  spread  over  the  ground 
for  such  ;i  purpose.    This  is  an  instance  of  the  too  frequent  tendency 

1  Thettaoompoqlng  KStSra  of  sodium  chlorid  on  cyanid  used  In  fumigation  was  fust  mentioned  by 
Bin  I  ii  1 1  lot  in  15,  Georgia  State  Board  of  Entomology,  1905. 


FUMIGATION  OF  CITRUS  TREES. 


Ol 


Ofl  many  writers  to  advise  a  practice  based  on  theory  alone.  The 
writer  has  scon  vegetation  destroyed  by  the  action  of  this  residue, 
especially  where  t lie  amount  of  residue  was  large.  Its  injurious 
effect  on  cover  crops  has  been  called  to  his  attention  by  one  orchard- 
ist.  One  of  the  most  striking  examples  of  its  injurious  effect  that 
the  writer  has  ever  seen  was  in  an  orchard  of  large  trees  on  light 
sandy  soil.  The  residue  was  emptied  at  the  trunks  of  the  trees,  with 
the  result  that  portions  of  the  root  systems  of  some  trees  were 
destroyed.  These  examples  go  to  show  that  the  residue  produce-  an 
immediate  injurious  effect  on  vegetation  wherever  it  may  come  into 
contact  with  the  same. 

The  residue  should  never  be  emptied  near  the  base  of  the  tree,  but 
out  in  the  middle  of  the  row.  It  should  be  so  placed  that  the  tents 
do  not  come  into  contact  with  it  while  being  moved  from  one  tree  to 
another.  The  common  practice  in  California  is  to  empty  it  midway 
between  the  two  rows  of  trees  in  the  opposite  direction  from  which 
the  tent-  ere  being  moved.  This  prevents  their  being  dragged  over 
the  residue. 

DOSAGES  FOR  VARIOUS  SCALE  PESTS. 

It  has  been  stated  previously  in  this  bulletin  that  the  purple, 
black,  red,  and  yellow  scales  were  the  insect  pots  of  citrus  trees 
against  which  fumigation  was  generally  practiced.  In  some  instances 
this  treatment  has  been  tried  against  the  mealy-bug.  The  distri- 
bution of  these  injects  is  such  that  in  one  locality  the  purple  scale 
might  be  the  principal  problem  of  control,  whereas  in  another  it 
might  be  the  black  scale,  red  scale,  or  yellow  scale.  Usually  a  single 
species  will  predominate  in  any  one  orchard,  yet  sometimes  two  or 
even  all  require  attention  at  the  same  time. 

The  tendency  of  citrus-fruit  growers  is  to  overlook  the  fact  that 
this  problem  of  control  is  one  wherein  different  species  of  insects  are 
concerned  and  to  believe  that  whatever  treatment  the  fumigator 
applies  should  accomplish  the  same  results  in  all  cases.  Only  the 
treatment  itself  is  considered,  not  the  strength  of  dosage,  and  this 
has  led  many  orchardists  to  complain  because  fumigation  by  the  out- 
fits owned  by  associations  or  counties  sometimes  costs  as  much  or 
even  a  little  more  than  that  by  contractors.  If  one  party  performs 
the  work  much  cheaper  than  another,  the  real  basis  of  this  cheapness 
is  that  less  cyanid  is  used.  The  desired  results  can  not  be  accom- 
plished unless  the  correct  dosage  requirements  are  met. 

Many  dosage  tables  for  the  different  scale  pests  of  citrus  trees 
have  been  published  in  California  and  elsewhere,  based  on  a  consid- 
eration of  the  height  and  width  of  the  trees.  These  dosage  tables  for 
the  most  part  are  very  erratic,  being  calculated  largely  from  hearsay 


52 


HYDROCYANIC- ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


rather  than  from  actual  experience  in  the  field.1  The  practice  among 
commercial  fumigators  has  been  to  absolutely  ignore  these  tables, 
depending,  instead,  on  their  own  judgment.  The  result  is  that 
their  scheduling   differs   markedly  from   that  of   the  published 

schedules. 

FACTORS  WHICH  AFFECT  THE  DOSAGE. 

Although  citrus-fruit  growing  in  southern  California  is  restricted 
to  a  limited  area  the  climatic  conditions  are  not  uniform  in  all  sec- 
tions. The  region  adjacent  to  the  coast  generally  is  cooler  and  much 
damper  at  night  than  in  the  interior  valleys.  This  situation  has  led  to 
a  diversity  of  opinion  among  fumigators  as  to  the  comparative 
dosage  for  the  two  sections.  Some  hold  that  a  heavier  dosage  is 
required  near  the  coast  on  the  ground  that  the  gas  is  absorbed  by 
the  dampness;  others,  that  the  drier  and  lighter  air  in  the  interior 
valleys  allows  a  more  rapid  escape  of  gas  through  the  tent,  which 
necessitates  more  eyanid  than  for  the  heavier  air  of  the  coast.  Set- 
ting aside  these  opinions  and  examining  the  situation  as  it  actually 
is.  we  find  that  the  general  dosage  strength  for  a  particular  insect  is 
approximately  the  same  throughout  southern  California  regardless 
of  nearness  to  or  remoteness  from  the  ocean.  Of  course,  there  are  a 
few  striking  individual  variations  from  this  general  statement,  but 
these  variations  are  as  noticeable  in  one  place  as  in  another. 

Persona]  experience  in  all  sections  has  taught  the  writer  that  the 
leakage  of  gas  is  for  the  most  part  noticeably  greater  in  the  dryer  and 
warmer  interior  sections  than  near  the  coast.  Despite  this. condition 
any  given  dosage  appears  to  be  as  efficient  in  one  place  as  in  another. 
This  marked  efficiency  in  the  dryer  and  warmer  sections  regardless 
of  the  greater  leakage  might  possibly  be  due  to  the  fact  that  the  scale 
insects  are  more  susceptible  to  the  gas  in  the  higher  temperatures 
general  there  than  in  the  cooler  temperatures  of  the  coast.  It  is 
known  among  entomologists  that  insects  are  active  at  high  tem- 
peratures but  become  dormant  at  low  temperatures  and  in  the  latter 
condition  are  more  difficult  to  destroy.  Prof.  Woodworth,  of  the 
I  niversity  of  California,  has  informed  the  writer  that  laboratory 
experiments  performed  by  him  have  shown  this  condition  to  exist 
among  scale  insects  and  that  the  temperature  at  which  they  become 
dormant  is  relatively  high.  This  interrelation  of  temperature  and 
activity  has  a  very  important  bearing  on  the  fumigation  treatment 
and  demands  much  further  experimentation  in  the  field  as  well  as  in 
the  laboratory. 

The  old  conception  that  an  increase  of  dosage  was  required  near 
the  coast  to  offset  the  loss  of  gas  from  absorption  by  moisture  is  also 
no  longer  tenable.     Experience  has  shown,  that  the  results  during 


1  Hul.  79,  liur.  Ent.,  U.  S.  Dept.  Agr.,  pp.  20-22,  19()9. 


V  I'MKiATION    OF   CITRUS  TREKS. 


53 


damp  oightB  near  the  coast  arc  exactly  as  satisfactory  as  on  dry  ones. 
Even  if  the  gas  is  absorbed  by  moisture  the  tents  become  so  much 
tighter  from  being  moist  that  any  negative  effect  from  the  dampness 
H  offset . 

The  character  of  the  tenting  material  used  directly  affects  the  dosage 
required,  teoai  of  the  ducks  and  drills  now  used  in  California  (see 
p.  11*  are  about  equally  gas-tight.  The  recommendations  of  dosage 
given  in  this  bulletin  are  for  these  tenting  materials.  With  the 
special  new  drill  experimented  with  during  these  investigations  (see 
pp.  11-12)  one-fourth  less  dosage  is  required.  Any  tenting  tighter 
than  the  cloth  commonly  used  for  this  purpose  in  California  will  also 
require  less  dosage. 

THE    PUBPLE  SCALE. 

Preliminary  experiments  to  determine  the  dosage  required  for  the 
destruction  of  the  purple  scale  were  undertaken  at  Orange,  Cal., 
during  the  month  of  November.  11)07.  Orange  trees  severely  infested 
with  the  purple  scale  in  all  stages  of  development  were  treated  with 
dosage  rates  varying  from  three-fourths  of  an  ounce  of  cyanid  per 
I  no  cubic  feet  up  to  2  J  ounces  per  100  cubic  feet.  The  cubic  contents 
of  the  trees  varied  but  little,  the  trees  ranging  from  11  to  14  feel  in 
height.  The  l  L-3  formula  was  followed.  Exposure  lasted  one  hour.  • 
After  a  period  of  about  two  months  an  examination  of  the  results  of 
this  experiment  was  made.  To  show  the  care  with  which  the  exam- 
ination was  conducted  in  this  as  well  as  all  other  experiments  against 
(lie  purple  scale  it  might  be  mentioned  that  in  each  case  the  scales 
were  overturned  and  examined  with  a  high-power  hand  lens.  In 
those  instances  in  which  the  entire  contents  of  the  scale  were  not  at 
once  revealed.  the  delicate*  ventral  scale  was  ruptured  and  the  con- 
sents scraped  out.  Through  this  method  not  a  single  egg  could 
escape  oba  nation. 

As  a  result  of  this  experiment  it  was  found  that  all  insects  were 
destroyed  on  the  leaves  and  branches  by  a  f-ounce  dosage  rate, 
that  all  insects  and  over  99  per  cent  of  the  eggs  were  destroyed  at  a 
I -ounce  dosage  rate,  and  that  all  eggs  on  the  leaves  and  branches  were 
destroyed  at  a  lA-ounce  dosage  rate.  Very  little  fruit  was  on  the  trees, 
yet,  where  present,  normal  eggs  were  found  on  the  fruit  after  a  dosage 
as  high  as  a  1  f-ounce  rate. 

In  another  experiment  in  which  the  trees  were  considerably  smaller, 
some  being  not  more  than  7  feet  tall,  it  required  a  2-ounce  rate  to 
eradicate  the  eggs  on  the  leaves  and  branches,  even  though  the  length 
of  exposure  was  one  and  one-half  hours.  This  condition  shows  that 
smaller  trees  require  a  much  heavier  dosage  proportionally  than  large 
trees  to  offset  the  leakage  of  gas,  as  explained  on  pages  33-34. 

During  July.  1908,  an  orange  orchard  near  Whittier,  consisting  of 
about  two  acre-  of  trees  averaging  about  7  to  \)  feet  tall  which  were 


54 


HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


severely  infested  with  purple  scale,  was  fumigated  with  the  1-1-3 
formula  with  an  exposure  of  one  hour.  Dosages  varying  from  1J 
to  2J  ounces  per  100  cubic  feet  were  used.  The  result  was  that 
eradication  occurred  on  the  leaves  and  branches  at  a  rate  of  2  ounces 
per  100  cubic  feet,  thus  corroborating  the  work  on  such  small  trees 
previously  carried  on  at  Orange. 

Eradication  of  an  insect  would  be  preferable,  yet  experience  during 
these  experiments  just  mentioned  demonstrated  that  the  dosage 
required  for  eradication  might  result  in  injury  to  the  fruit.  A  rate 
of  1  ounce  per  100  cubic  feet  for  trees  about  1 1  to  12  feet  high  was  safe 
under  almost  all  conditions  and  such  a  rate  was  adopted  for  general 
work.  Dosage  schedule  No.  1,  based  on  such  a  strength  of  gas,  was 
prepared  at  this  time. 

This  schedule  No.  1  was  used  during  the  autumn  of  1908  by  several 
practical  outfits  in  the  vicinity  of  Whit  tier,  and  has  since  been  fol- 
lowed in  other  sections  in  which  the  purple  scale  occurs.  Thousands 
of  acres  have  been  fumigated  after  this  schedule.  The  general  result, 
when  the  work  has  been  carefully  done,  has  corroborated  the  writer's 
own  experiments  in  that  all  live  insects  and  in  excess  of  99  per  cent 
of  the  eggs  were  destroyed  on  the  leaves  and  branches.  Such  a 
killing  is  entirely  satisfactory.  In  some  cases  a  slight  amount  of 
pitting  of  fruit  has  occurred,  especially  in  the  top  of  the  trees,  and  has 
caused  some  growers  to  complain.  This  slight  amount  of  pitting  can 
be  overlooked  by  reason  of  the  superior  killing  which  has  resulted. 
To  use  a  dosage  sufficient  to  control  the  scale  and  at  the  same  time 
entirely  avoid  pitting  throughout  a  fumigating  season  is  a  practical 
impossibility. 

The  first  season  the  improved  system  of  fumigation  was  adopted 
schedule  No.  1  was  used  almost  universally.  As  this  schedule  gives 
dosages  considerably  in  excess  of  those  formerly  used,  fumigators  in 
general  became  somewhat  uneasy  about  using  it,  with  the  result  that 
during  the  season  of  1909  a  three-fourths  schedule,  rather  than  full 
schedule  No.  1,  was  used  by  the  majority  of  outfits.  Although  the 
results  with  the  three-fourths  schedule  have  been  very  good,  this 
schedule  is  far  less  satisfactory  than  the  full  schedule  No.  1. 

The  writer  advises  the  use  of  full  schedule  No.  1  (see  fig.  9,  p.  34) 
for  the  purple  scale.  The  results  with  this  dosage  are  so  superior,  as 
shown  by  experience,  that  most  orchards  are  rendered  so  clean  that 
thereafter  they  do  not  require  fumigation  oftener  than  once  in  two 
years.  It  is  more  economical  to  use  schedule  No.  1  and  escape  treat- 
ment alternate  years  even  if  a  little  fruit  is  pitted  in  the  operation 
than  to  use  a  smaller  dosage  and  he  obliged  to  treat  an  orchard  every 
year.  It  is  seldom,  however,  that  any  marked  degree  of  pitting  takes 
place  with  schedule  Xo.  J,  it'  proper  care  is  exercised  during  the 
operation. 


FUMIGATION  OF  CITRUS  TEEES. 


55 


IKNCTH  OF  EXPOSURE. 

Experiments  against  the  ])urple  scale  showed  that  in  using  a  2-ounce 
dosage  rate  eradication  occurred  on  the  leaves  and  branches  with  a 
30-minutes  exposure,  whereas  with  a  one-hour  exposure  it  was  pos- 
sible to  accomplish  the  same  results  by  using  a  H-ounce  dosage  rate. 
This  demonstrates  that  decidedly  better  results  can  be  secured  by 
leaving  the  tents  on  the  trees  one  hour  than  is  possible  with  30 
minutes  of  gassing.  With  the  present  character  of  tents  in  use 
practically  all  gas  has  escaped  on  most  nights  by  the  end  of  an  hour. 
This  furnishes  sufficient  evidence  that  a  longer  exposure  would  be 
unnecessary.  However,  experiments  have  been  carried  on  in  which 
exposures  of  greater  duration  than  one  hour  were  made,  but  no 
better  killing  resulted.  From  all  the  experimental  evidence  at  hand, 
an  exposure  of  one  hour  is  advised  for  the  purple  scale.  This  length 
of  time  readily  enables  an  outfit  to  go  through  the  complete  operation 
of  preparing  the  chemicals  and  dosing  the  trees,  with  a  few  minutes 
to  spare  for  resi . 

ERADICATION. 

Experiments  during  the  earlier  part  of  the  investigation  showed  that 
the  purple  scale  could  be  eradicated  from  the  leaves  and  branches  of 
trees  bv  using  a  dosage  equivalent  to  a  H-dosage  schedule.  During 
the  first  part  of  September,  190S,  an  isolated  orange  orchard  containing 
about  1  acre  of  trees  from  10  to  IS  feet  tall  and  severely  infested  with 
the  purple  scale  was  fumigated,  using  a  1  \-dosage  schedule  (dosage 
schedule  No.  1  increased  one-half).  No  old  scaly  fruit  was  left  on  the 
trees.  The  results  were  as  follows:  An  inspection  of  this  orchard 
during  the  latter  part  of  the  autumn  failed  to  reveal  any  live  insects. 
The  crop  of  fruit  on  the  trees  was  entirely  free  of  scale  for  the  first  time 
in  the  memory  of  the  owner.  Many  examinations  have  been  made 
since,  yet  without  the  finding  of  a  single  live  insect. 

This  experiment  has  shown  that  eradication  of  the  purple  scale  on 
tree-  free  of  infested  fruit  is  possible  with  a  H-dosage  schedule,  if 
the  work  be  carefully  done.  In  small  isolated  orchards  it  might  be 
practicable  at  certain  times  to  use  this  dosage.  For  general  work 
the  employment  of  this  eradication  dosage  is  not  advised.  The 
Writer's  experience  has  assured  him  that  careful  work  under  the  most 
favorable  conditions  would  largely  avoid  pitting  of  fruit  even  with 
this  high  dosage.  But,  as  a  matter  of  fact,  the  work  in  the  field  is  not 
always  carefully  done,  nor  are  the  most  favorable  conditions  always 
taken  advantage  of.  Experience  has  shown  that  the  pitting  of  fruit 
with  regular  schedule  No.  1  sometime-  causes  a  Blight  dissatisfaction 
among  growers.  If  the  injury  from  schedule  No.  1  sometimes  pro- 
duces dissatisfaction,  it  is  very  evident  that  the  greater  risk  with  a 
larger  dosage  is  too  great  to  justify  its  general  adoption. 
67330°— Bull.  90—12  5 


56  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

Other  conditions  exist  which  take  part  in  prohibiting  this  greater 
dosage.  If  this  dosage  were  used  in  general  orchard  work,  it  is  doubt- 
ful if  eradication  would  occur  in  all  cases.  Tents  not  properly 
pulled  down  on  all  sides  of  the  tree,  a  hole  in  the  tent,  mistake  in 
measuring  the  trees  or  in  reading  the  dosage  from  the  schedule, 
erroneous  measuring  of  chemicals,  boiling  over  of  a  generator,  over- 
turning of  a  generator,  and  numerous  other  considerations  which  will 
sometimes  escape  even  the  most  careful  manipulator,  make  the  differ- 
ence between  eradication  and  noneradication  more  variable  in  prac- 
tice than  in  theory.  If  the  fumigator  is  inclined  to  be  a  little  careless, 
some  of  the  above  errors  will  frequently  creep  in. 

Moreover,  unless  compelled  to  do  so  the  orchardists  in  any  one 
locality  would  not  all  use  this  dosage,  while  possibly  some  would  not 
fumigate  at  all.  To  go  to  the  extra  expense  required  in  an  eradication 
dosage  and  then  be  subject  to  reinfestation  from  one's  neighbors 
presents  no  special  attractiveness  to  the  grower.  Supposing  that 
the  growers  in  any  one  locality  were  willing  to  use  an  eradication 
dosage,  the  present  number  of  fumigation  outfits  is  inadequate  to 
meet  this  requirement  within  the  limited  time  necessary  in  order  to 
prevent  reinfestation.  These  practical  considerations  demonstrate 
that  the  eradication  of  the  purple  scale  from  any  large  district  is  im- 
practicable at  the  present  time. 

DIFFICULTY  OF  DESTROYING  THE  SCALE  ON  THE  FRUIT. 

There  is  one  more  important  point  which  must  be  considered  in 
connection  with  fumigation  for  the  purple  scale.  In  experiments 
to  which  attention  has  been  called  it  has  been  shown  that  destruction 
of  scale  is  much  more  difficult  on  the  fruit  than  on  the  leaves  and 
branches.  Careful  investigation  of  this  point  for  about  two  years 
has  also  shown  that  the  susceptibility  of  the  scale  on  some  fruit  is 
much  greater  than  on  others.  Hence,  no  exact  standard  of  destruc- 
tion for  the  scale  on  fruit  is  possible.  When  scales  become  matured 
and  deposit  eggs  the  dosage  required  for  eradication  is  very  much 
greater  on  the  fruit  than  on  the  leaves  and  branches.  It  may  require 
a  one-fourth  to  one-half  or  in  some  cases  an  even  greater  increase. 
A  dosage  sufficient  for  eradication  of  the  scale  on  the  fruit  is  impracti- 
cal for  the  very  same  reasons  that  make  eradication  on  the  leaves 
and  I) ranches  commercially  impractical.  A  grower  possessing  a 
few  t  rees  on  which  lie  intends  to  eradicate  the  scale  at  one  fumigation 
should  remove  all  infested  fruit  before  the  operation  and  then  use  a 
1  \  schedule.  It  is  advisable  to  remove  the  old  scaly  fruit  in  any  fumi- 
gation.  At  picking,  fruit  badly  infested  with  scale  is  usually  left  on 
the  tree,  and  frequently  from  one  to  a  half  dozen  or  more  old,  scale- 
infested  oranges  per  tree  remain  throughout  an  orchard.  Even  after 
a  good  fumigation  one  of  these  old  fruits  might  carry  more  healthy 


FUMIGATION  OF  CITRUS  TREES. 


57 


purple-scale  eggs  than  all  the  rest  of  the  tree,  and  on  the  hatching 
of  these  eggs  the  insects  will  spread  to  other  parts  of  the  tree.  The 
danger  from  old  scaly  fruit  is  evident,  and  all  such  should  be  re- 
moved from  the  trees  before  fumigating  an  orchard. 

There  are  times  in  which  a  scale-infested  orchard  to  be  treated 
contains  some  scale  on  the  green  fruit.  During  the  autumn  season 
when  fumigation  is  most  practiced  the  purple  scale  is  largely  in  its 
earlier  stages  of  development,  in  which  it  may  be  destroyed  by  the 
employment  of  schedule  No.  1.  The  immature  fruit  which  is  scale 
infested  can  be  left  on  the  tree.  It  is  the  old  scaly  fruit  which  requires 
removal  at  the  time  of  fumigation. 

TWO   Sl((  KSSIVF.  TRF.ATMKN  TS  . 

A  few  growers  whose  groves  are  severely  infested  with  the  purple 
scale  will  desire  to  have  the  scale  eradicated  if  possible,  even  though 
the  initial  expense  is  considerably  above  the  cost  of  a  regular  treat- 
ment, yet  they  do  not  care  to  assume  the  risk  of  having  any  fruit 
on  the  trees  injured.  In  such  cases  some  authorities  advise  two 
successive  treatments  during  the  early  autumn  and  about  five  or  six 
weeks  apart.  The  <|.>-;il;<'^  used  should  !><'  Sufficient  t«»  destroy  the 
mature  insects.  The  first  treatment  would  destroy  all  the  insects, 
leaving  only  eggs  on  the  trees.  The  time  elapsing  between  this  and 
the  second  treatment  should  be  just  long  enough  to  allow  all  the  eggs 
to  hatch.  About  live  weeks  is  supposed  to  be  sullicient  unless  the 
weather  be  exceptionally  cool.  Careful  inspection  will  settle  this 
point.  If  the  first  treatment  has  been  thorough  and  there  are  no  eggs 
present  at  the  second,  eradication  should  result.  A  three-fourths 
schedule  should  be  used  in  each  treatment.  The  first  fumigation 
should  be  in  the  autumn,  not  later  than  the  first  part  of  October. 
Double  fumigation  is  seldom  resorted  to,  as  its  economy  in  the  long 
run  is  somewhat  questionable. 

THE   RED  SCALE. 

The  red  scale  is  generally  held  as  the  most  difficult  of  all  citrus  scales 
to  destroy.  Extensive  experiments  during  this  investigation,  carried 
out  in  many  sections  of  southern  California,  have  proved  it  to  be  one 
of  the  easiest  to  destroy.  It  is,  however,  the  most  difficult  insect  to 
keep  out  of  an  orchard  when  once  it  has  become  established  in  a  com- 
munity, and  this  may  be  the  basis  for  the  opinion  as  to  its  greater 
resistant  power  to  hydrocyanic-acid  gas.  By  reason  of  its  great  pro- 
lificness,  its  infestation  of  some  weeds  common  about  citrus  orchards 
as  well  as  many  trees  and  shrubs  which  are  sometimes  planted  on 
driveways  or  about  the  buildings  on  the  premises,  and  the  ease  with 
which  it  spreads,  this  insect  frequently  will  quickly  reinfest  an  orchard 
which  has  been  treated.  Live  insects  left  on  a  few  trees  in  an  orchard 
quickly  multiply  and  infest  the  others.    The  author  has  eradicated 


58  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

the  red  scale  in  orchards  and  yet  these  have  become  reinfested  within 
a  year.  In  one  orchard  the  reinfestation  was  traced  to  some  fig  trees 
on  one  side  which  had  not  been  fumigated;  in  another,  the  scale 
spread  from  a  neighboring  orchard  across  the  way;  while  in  a  third, 
the  scale,  came  from  nightshade  (Solarium  sp.)  which  had  not  been 
destroyed.  The  insect  is  distributed  by  the  wind,  by  birds,  and 
especially  by  clinging  to  the  bodies  of  the  hordes  of  insects  which 
frequent  citrus  trees  and  carry  them  to  other  trees.  Foremost  among 
these  insects  are  the  ladybirds  (Coccinellidae),  of  which  there  are 
numerous  species  as  well  as  vast  quantities  of  individuals.  Before 
fumigating  for  the  red  scale  care  should  be  taken  that  host  weeds 
along  irrigation  flumes,  ditches,  and  fences  are  destroyed  so  far 
as  possible  and  all  neighboring  trees  subject  to  its  attacks  cleaned  up. 

Dosage. — The  first  orchard  treated  for  the  red  scale  (December, 
1907)  was  a  severely  infested  one  of  between  2  and  3  acres  of  trees 
in  an  unhealthy  condition,  and  was  located  at  Sierra  Madre.  The 
height  of  the  trees  was  about  10  to  14  feet.  The  1-1-3  formula 
was  used.  Exposure  lasted  one  hour.  Dosages  of  from  one-half 
to  3  ounces  per  100  cubic  feet  were  used.  Eradication  took  place 
with  all  strengths. 

In  September,  1908,  about  1  acre  of  trees  about  10  feet  tall, 
located  at  Whittier,  was  treated  with  dosages  of  from  of  1  to  1  j  ounces 
per  100  cubic  feet.    Exposure  lasted  one  hour.    Eradication  resulted. 

During  April,  1909,  4  to  5  acres  of  unhealthy  orange  and  lemon 
trees  at  Villa  Park,  Orange  County,  were  fumigated  with  dosages  of 
from  one-half  to  1J  of  schedule  No.  1.  The  exposures  lasted  45 
minutes,  1  hour,  and  1J  hours.    Complete  eradication  occurred. 

An  acre  of  entirely  healthy  orange  trees  severely  infested  with  the 
red  scale — fruit  as  well  as  leaves  and  branches — was  treated  during 
September,  1909.  The  results  of  this  experiment  showed  that  a 
one-half  schedule  usually  would  destroy  the  scale  on  the  leaves  and 
branches,  but  that  it  required  a  three-fourths  schedule  to  accomplish 
this  on  the  fruit.  As  satisfactory  work  was  done  with  an  exposure 
of  45  minutes  as  with  1  hour. 

The  examination  of  much  work  carried  on  by  practical  outfits  using 
both  a  three-fourths  schedule  and  a  No.  1  schedule  has  demonstrated 
thai  eradication  would  result  when  careful  work  was  done. 

Results  from  these  extensive  observations  show  that  the  red  scale 
is  more  easily  destroyed  on  unhealthy  than  on  healthy  trees,  and 
that  it  is  slightly  more  difficult  to  destroy  on  the  fruit  than  on  the 
leaves  and  branches.  The  dosage  used  must  be  based  on  a  strength 
sufficient  to  destroy  the  scale  on  all  parts  of  all  trees;  thus  it  is 
apparent  that  a  three-fourths  schedule  is  the  most  economical  for 
the  red  scale.  In  all  fumigation  work  against  this  insect  it  is  advised 
thai  a  three-fourths  schedule  (three-fourths  of  schedule  No,  1)  (see 
fig.  11)  be  used.    An  exposure  of  45  minutes  is  sufficient. 


FUMIGATION  OF  CITBUS  TREES. 


59 


THE   BLACK  SCALE. 

To  specify  a  certain  dosage  for  use  at  all  times  against  the  black 
scale  is  impractical.  The  reason  is  that  this  insect  is  more  difficult 
to  destroy  in  some  stages  of  its  development  than  in  others.  While 
young  and  in  a  soft  condition  it  can  be  destroyed  by  a  light  dosage. 
As  the  insed  approaches  maturity  its  body  becomes  leathery  and 
tough,  which  renders  it  difficult  to  destroy.  The  eggs  require  even 
a  heavier  dosage.  The  great  variation  in  development  among  the 
black  scale  results  that  at  most  times  of  the  year  trees  will  contain 
insects  in  all  stages  of  development,  from  those  recently  hatched  to 


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tough  or  even  leathery  ones.  In  such  cases  a  dosage  sufficient  to 
destroy  the  more  resistant  individuals  is  necessary. 

Experiments  during  this  investigation,  as  well  as  observation  of  the 
work  of  practical  outfits,  has  demonstrated  that  in  the  younger  and 
tenderer  stages  this  insect  can  be  destroyed  by  one-half  of  schedule 
Ko.  1.  Very  few  of  those  in  the  later  stages  of  development  are 
affected  by  such  a  dosage.  A  three-fourths  schedule  not  only 
destroyed  those  in  the  earlier  stages  of  development,  but  also  those 
of  a  leathery  nature  and  many  that  are  tough  and  full  grown,  as 
evidenced  by  their  size,  together  with  the  absence  of  the  character- 
istic Letter  UH"  on  the  upper  side.  A  considerable  percentage  of 
eggs  is  also  destroyed  at  this  strength.    A  full  schedule  No.  1  dosage 


60  HYDROCYANIC- ACID  GAS  FUMIGATION  IX  CALIFORNIA. 


will  destroy  all  insects  except  a  few  of  the  toughest  of  the  matured 
scales.    It  will  also  destroy  a  large  percentage  of  the  eggs. 

In  fumigating  for  the  black  scale  the  treatment  should  be  applied  at 
the  time  when  the  insects  are  largely  or  all,  if  possible,  in  the  early  stages 
of  development.  Close  watch  of  the  condition  of  the  scale  in  his 
orchard  will  enable  the  grower  to  determine  the  proper  time.  There 
is  a  general  breeding  period  during  late  summer  or  early  autumn 
when  this  condition  usually  exists.  Wait  until  the  eggs  of  the  old 
brood  have  all  hatched,  even  if  by  this  time  some  of  the  earliest 
issuing  insects  are  becoming  leathery.  This  is  the  time  at  which 
fumigation  is  advised  and  a  three-fourths  schedule  (see  fig.  11,  p.  59) 
should  be  used. 

If  it  be  desirable  to  fumigate  at  a  time  when  the  insects  are  in  all 
stages,  including  the  mature  and  tough  individuals  as  well  as  those 
of  the  younger  generations,  a  full  schedule  Xo.  1  (see  fig.  9,  p.  34) 
should  be  used. 

It  would  be  difficult  to  advise  a  dosage  sufficient  to  destroy  the 
eggs,  because  of  the  wide  variability  in  the  action  of  such  a  dosage. 
The  writer  has  seen  the  eggs  of  some  species  of  scale  insects  destroyed 
by  employing  schedule  No.  1,  whereas  in  other  instances  the  eggs 
have  been  unaffected  even  where  a  very  much  heavier  dosage  was 
used.  This  is  partly  attributable  to  the  closer  attachment  to  the 
host  plant  of  the  different  individual  scales  and  partly  to  a  small 
parasitic  insect  (ScuteTlista  cyanea  Motsch.)  which  attacks  the  eggs 
of  the  black  scale  very  freely.  The  larva?  of  this  parasite  cements 
the  edges  of  the  scale  to  the  host  plant,  making  it  more  difficult  for 
the  gas  to  penetrate  to  the  eggs  under  such  scales  than  under  those 
not  parasitized.    The  exposure  should  last  one  hour. 

THE  YELLOW  SCALE. 

The  yellow  scale  is  comparable  in  almost  all  respects  with  the  red 
scale  except  that  it  is  much  less  destructive.  The  region  in  and  about 
liedlands,  San  Bernardino  County,  is  the  principal  section  in  southern 
California  in  which  special  effort  for  the  control  of  this  insect  alone 
is  required. 

During  September,  1909,  2  to  3  acres  of  orange  trees  at  Redlands 
infested  with  the  yellow  scale  were  treated  with  dosage  rates  of  from 
one-half  to  1%  of  schedule  No.  1.  The  insects  were  destroyed  on  all 
trees  except  those  on  which  a  one-half  schedule  was  used.  A  very 
few  survived  on  these.  No  difference  in  effect  was  apparent  between 
an  exposure  of  45  minutes  and  one  of  an  hour. 

A  three-fourths  schedule  (see  fig.  11,  p.  59)  is  recommended  for 
the  yellow  scale.  The  section  in  which  the  yellow  scale  is  most 
serious  is  about  Redlands,  where  a  great  acreage  is  on  terraced  land. 
In  such  fumigation  it  is  believed  that  a  full  schedule  No.  1  should  be 


FUMIGATION  OF  CITRUS  TREES. 


61 


used,  to  make  sufficient  allowance  for  the  irregularity  in  the  surface 
of  the  ground,  which  renders  the  possibility  of  leakage  from  under- 
neath 'neater  than  in  regular  orchard  work. 

DOSAGES  IX  GENERAL  FUMIGATION. 

If  the  treatment  is  for  any  one  of  the  insects  mentioned  previously, 
the  dosage  recommended  for  that  particular  insect  should  be  used. 
Frequently,  however,  two,  or  even  three,  different  species  may  he 
found  in  the  same  grove.  In  such  cases  use  that  dosage  which  is 
heavy  enough  to  destroy  the  most  resistant  one.  For  instance,  if 
the  purple  and  red  scales,  or  the  purple  and  black  scales,  occur  in 
the  same  orchard  use  dosage  schedule  No.  1  for  an  exposure  of  one 
hour.  If  the  red  and  black  scales,  or  the  yellow  and  black  scales, 
occur  .simultaneously,  use  a  three-fourths  schedule  for  one  hour, 
unless  the  black  scale  is  in  an  advanced  or  matured  stage,  in  which 
case  a  full  No.  1  schedule  is  required. 

TIME  OF  THE  YEAR  FOR  FUMIGATION. 

Although  fumigation  is  carried  on  in  California  at  all  times  of  the 
▼ear,  there  are  certain  periods  in  which  the  operations  are  more 
general.  There  are  two  main  factors  to  be  taken  into  consideration 
in  fumigating,  i.  e.,  the  species  of  scale  insect  and  the  condition  of 
the  tree.  As  to  the  latter,  it  may  be  said  that  at  certain  periods  of 
the  year  the  fruit  is  in  such  a  tender  condition  that  it  can  not  with- 
stand a  heavy  dosage  without  injury.  This  period  is  while  the  fruit 
is  of  small  size,  usually  from  April  until  about  August.  The  bulk  of 
fumigation  in  California  at  the  present  time  is  carried  on  between  the 
latter  part  of  August  and  December.  Probably  the  principal  reason 
for  treating  during  this  period  is  that  the  black  scale  is  usually  at 
that  time  most  successfully  reached.  Although  the  life  history  of 
the  black  scale  has  never  been  thoroughly  worked  out  for  the  region 
in  which  these  investigations  were  made,  it  is  generally  understood 
that  the  majority  of  the  insects  of  the  largest  and  most  regular  brood 
are  hatched  and  in  their  least  resistant  stage  during  the  months  of 
September  and  October.  In  some  favorable  seasons  the  eggs  are 
almost  all  hatched  in  August . 

The  black  scale  occurs  in  practically  every  citrus-growing  locality 
in  southern  California,  while  the  purple,  red,  and  yellow  scales  are 
more  localized.  Where  any  of  these  other  species  occur  in  orchards 
infested  with  the  black  scale  it  is  a  common  practice  to  fumigate 
during  the  regular  black-scale  period,  using  the  dosage  necessary  to 
destroy  the  most  resistant  species.  The  majority  of  these  scale 
insects  can  thus  be  caught  at  one  time.  When  fumigating  for  the 
purple  scale  alone,  operations  can  be  commenced  as  early  in  the 
season  as  the  trees  are  in  a  condition  to  withstand  the  heavy  dosage 


62 


HYDROCYANIC- ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


required  without  injury  to  the  fruit,  although  probably  it  would  be 
preferable  to  fumigate  a  little  later  in  the  fall.  The  purple  scale  can 
be  found  in  the  egg  stage  throughout  the  year.  There  is,  however, 
a  period  in  the  fall  during  which  the  smallest  proportion  of  eggs  is 
to  be  found.  With  dosages  lower  than  that  of  eradication  the  best 
results  can  be  accomplished  at  this  time,  which  usually  is  somewhere 
about  October.  The  red  and  yellow  scales  are  born  alive  and  can 
be  successfully  destroyed  throughout  the  year. 

In  fumigating  for  any  of  the  scale  insects  there  is  one  point  worthy 
of  consideration.  Aside  from  trying  to  save  the  tree  from  destruc- 
tion or  from  having  its  vitality  impaired  by  the  attack  of  scale  pests, 
the  orchardist  fumigates  principally  in  order  to  have  his  fruit  come 
into  the  packing  house  as  clean  as  possible.  It  would  be  well,  there- 
fore, to  fumigate  as  nearly  as  possible  at  the  time  which  would  insure 
him  the  cleanest  fruit.  Although  lemons  are  gathered  throughout 
the  entire  year,  the  bulk  of  the  orange  crop  is  taken  during  the  first 
six  months.  Thus  fumigation  during  the  fall  and  early  winter 
would  be  sure  to  place  the  cleanest  fruit  in  the  packing  house.  If 
carried  on  in  the  late  spring  or  early  summer,  such  insects  as  remain 
undest roved  would  have  the  opportunity  to  breed  throughout  a  period 
of  several  months  and  infest  much  fruit. 

The  small  size  of  the  fruit  and  to  some  extent  the  heat  renders 
fumigation  generally  unsafe  before  the  month  of  August.  From 
this  time  up  to  December  the  weather  is  usually  quite  dry  and  not 
especially  cold.  December  is  usually  the  month  during  which  cold, 
rainy,  and  generally  unsettled  wTeather  may  be  expected  to  com- 
mence and  to  continue  with  more  or  less  irregularity  into  the  follow- 
ing March.  Serious  injury  to  the  trees  may  result  unless  the  greatest 
care  is  exercised  in  treating  during  these  winter  months.  At  this 
period  most  fruit  is  almost  fully  grown,  which  adds  a  great  weight 
to  the  tree.  Covering  heavily  laden  trees  with  fumigation  tents  not 
only  tears  off  and  scars  a  considerable  amount  of  fruit,  but  also 
breaks  off  heavy  branches.  The  writer  does  not  believe  it  advisable 
to  treat  trees  in  such  condition. 

The  labor  question,  at  all  times  a  perplexing  one  in  fumigation 
operations,  is  especially  serious  where  the  work  is  carried  on  in  the 
winter.  The  frequent  delays,  possibly  for  several  days  at  a  time, 
necessitated  by  the  rainy  and  damp  weather  render  it  difficult  to 
secure  good  and  careful  men  who  will  remain  continuously  with  the 
outfits.  As  they  are  paid  only  for  actual  work  performed,  operators 
earn  little  more  than  their  living  expenses  during  the  irregular  hours 
when  work  is  possible  in  the  winter  season. 

After  viewing  all  sides  of  the  question,  it  is  advised  that  the  prefer- 
able months  for  general  fumigation  are  from  August  to  December. 
The  treatment  can,  however,  be  carried  on  with  both  safety  and 


FUMIGATION  OF  CITRUS  TREES. 


63 


efficiency  between  December  and  April,  provided  the  work  be  per- 
formed by  careful  men  who  observe  the  various  factors  affecting 
these  two  considerations.  These  factors  are  given  at  various  places 
throughout  this  bulletin. 

FUMIGATION  FOR  THE  MEALY  BUG. 

( rOnaiderable  excitement  has  been  aroused  in  some  parts  of  southern 
California  dining  the  last  year  or  so  over  a  so-called  threatened 
invasion  of  the  citrus  mealybug  (Pseudococcus  citri  Kisso).  This 
insect  has  been  known  in  this  region  for  at  least  15  years.  Its 
greatest  injury  has  been  done  in  the  vicinity  of  National  Cit}  ,  San 
Diego  County;  along  the  Sierra  Madre  foothills,  near  Monrovia,  Los 
Angeles  County  ;  and  very  recently  at  Santa  Paula,  Ventura  County. 
There  have  also  been  sporadic  occurrences  in  many  other  sections. 
This  sporadic  activity  has  been  of  a  somewhat  puzzling  nature.  A 
tree  severely  infested  with  the  mealy  bug  one  day  might  be  found  to 
be  practically  free  of  this  insect  a  month  later,  though  no  artificial 
measures  of  control  had  been  applied.  The  mealybug  might  reap- 
pear on  this  same  tree  the  following  year,  or  even  sooner.  The 
writer  is  inclined  to  attribute  this  disappearance  largely  to  the 
activity  of  parasitic  and  predaceous  insect  enemies.  The  general 
appearance  of  the  remains  of  the  mealy  bug.  as  well  as  the  rear- 
ing of  several  beneficial  insects  therefrom,  including  two  or  three 
species  of  I  Ivmenoptera,  a  brown  lacewing  (Hemerobius  sp.)i  and  a 
syrphus  fly,  lead  the  writer  to  this  conclusion.  Several  species  of 
Coccinellida-  also  have  been  found  present  in  severe  infestations  of 
this  insect.  Undoubtedly  climatic  conditions  as  well  as  fungous  and 
bacterial  diseases  produce  some  effect. 

The  recent  prominence  of  this  insect  in  the  immediate  vicinity  of 
Santa  Paula.  Ventura  County,  is  well  worthy  of  mention.  Its  infes- 
tation here  lias  been  so  severe  that  the  proportion  of  fruit  in  sonic 
orchards  ruined  by  this  insect  during  1909  was  very  great.  The 
huge  amount  of  damage  caused  there,  together  with  the  difficulties 
experienced  in  destroying  the  mealy  bug,  has  led  to  agitation  for  its 
control  in  several  other  localities. 

At  various  times  during  this  investigation  a  few  trees  infested  with 
mealy  bugs  have  been  treated.  The  usual  dosage  has  been  I  V  to  2 
t^mes  schedule  Xo.  1.  The  results  have  been  variable.  Some  trees 
would  appear  to  be  entirely  freed,  while  on  others  many  live  insects 
would  remain. 

Dining  the  autumn  of  1909  a  prominent  orchardist  of  Santa  Paula 
carried  on  some  very  extensive  fumigation  work  against  this  insect, 
partly  according  to  the  writer's  directions.  Some  trees  were  treated 
with  2,  2\,  and  3  times  schedule  Xo.  1  for  from  one  to  one  and  one- 
half  hours  without  eradicating  the  scale  in  any  case.    The  results 


64  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


with  these  different  dosages  were  about  the  same.  The  very  young 
insects  were  destroyed,  but  a  large  proportion  of  the  matured  ones 
and  eggs  escaped.  A  few  trees  were  then  covered  with  double  tents 
(two  tents,  one  over  the  other)  in  order  to  render  them  tighter  and 
twice  schedule  No.  1  was  used.  An  examination  of  these  results  a  few 
days  after  treatment  showed  eradication  on  some  trees,  while  others 
contained  a  small  percentage  of  live  adult  insects.  A  10-acre  block 
of  trees  was  then  fumigated  with  double  tents  (two  tents,  one  over 
the  other)  and  twice  schedule  Xo.  1.  An  examination  of  this  work 
showed  a  considerable  percentage  of  live  matured  insects  and  eggs. 

From  the  results  of  the  aforementioned  fumigation  against  the 
mealy  bug  it  is  seen  that  the  early  stages  of  this  insect  are  destroyed 
by  the  use  of  twice  dosage  schedule  Xo.  1.  If  a  study  of  the  life 
history  should  reveal  that  at  some  certain  time  they  may  all  be 
found  in  this  early  stage  of  development  fumigation  might  then  prove 
of  some  avail.  The  failure,  however,  to  secure  eradication  of  the 
mealy-bug  in  its  more  mature  stages,  even  where  dosages  as  high  as 
three  times  schedule  Xo.  1  were  used,  indicates  that  general  com- 
mercial fumigation  for  the  mealy  bug  is  impractical.1 

FUMIGATION  DURING  THE  BLOSSOMING  PERIOD. 

The  statements  by  experts  on  fumigation  as  to  the  amount  of 
injury  resulting  from  work  while  the  trees  are  in  blossom  are  very 
conflicting.  Some  fumigators  hold  that  a  very  light  dosage  will 
destroy  tender  blossoms,  while  others  believe  that  the  blossoms  will 
stand  a  heavy  dosage.  In  order  to  decide  this  point,  much  experi- 
mentation has  been  carried  on  and  many  observations  made  through- 
out this  investigation.  To  attempt  to  relate  the  details  of  the  scores 
of  experiments  and  observations  made  along  this  line  in  all  parts  of 
southern  California  would  require  too  much  space,  so  mention  will 
be  made  here  only  of  general  results.  Fumigation  observations  by 
the  writer  have  covered  the  entire  period  from  the  time  the  blossoms 
first  appear  until  the  petals  drop.  In  not  a  single  instance  during  this 
period  has  any  serious  injury  resulted  even  though  dosages  as  high  as 
1 J  and  2  times  schedule  No.  1  have  sometimes  been  used. 

Occasionally  some  blossoms  were  affected,  and  dropped  off  soon 
after  the  treatment,  yet  these  blossoms  were  normally  weak  and 
would  have  fallen  without  fumigation.  The  fumigation  merely 
hastened  their  shedding.  Even  if  a  heavy  shedding  took  place 
(which  never  happens),  there  would  be  no  cause  for  alarm,  as  the 
getting  of  only  a  small  percentage  of  the  blossoms  on  a  citrus  tree  is 
necessary  to  produce  a  full  crop  of  fruit . 


1  This  Mine  conclusion  has  been  reached  by  Mr.  E.  O.  Essig,  horticultural  commissioner  of  Ventura 
Count  v.    i  Sec  I  'ouiona  College,  Journal  ol  Entomology,  Decern  her,  1909.) 


FUMIGATION   OF  CITRUS  TREES. 


05 


The  blossoms  appear  to  be  much  more  resistant  to  the  gas  than 
the  young  leaves  or  leaf  shoots.  Trees  in  which  there  are  blossom 
shoots  and  tender  leaf  shoots  side  by  side  will  have  the  leaf  shoots 
burned  back,  while  the  blossoms  remain  unaffected.  This  shows  that 
the  blossoms  w'lV  stand  a  much  heavier  dosage  than  the  tender  leaves 
and  leaf  shoots. 

The  vomit:  leaf  shoots  grow  so  rapidly  in  certain  varieties  of  trees, 
or  even  in  tin1  same  variety  of  tree  in  different  localities,  that  during 
the  latter  part  of  the  blossoming  period  they  almost  obliterate  the 
blossoms.  Although  fumigation  of  these  trees  will  not  injure  the 
blossoms  it  will  frequently  burn  back  these  leal  blanches  very  severely. 
In  such  cases  the  grower  should  not  become  alarmed  by  the  burning, 
as  his  trees  and  future  crop  of  fruit  are  in  no  way  endangered.  Re- 
newed growth  will  soon  take  place, while  the  crop  of  fruit  will  be 
exactly  as  large  as  if  the  trees  were  untreated. 

In  conchision.it  might  be  said  that  experience  in  this  investigation 
has  shown  that  fumigation  can  be  safely  conducted  during  the 
blossoming  period  with  the  dosages  at  present  generally  employed 
by  fumigators,  namely,  schedule  No.  1. 

FUMIGATION  WHILE  THE  FRUIT  IS  OF  SMALL  SIZE. 

The  records  of  several  experiments  during  1908  in  fumigating 
while  the  fruit  was  of  small  size  are  given  in  Bulletin  79  of  this  bureau. 
The  results  of  these  experiments  demonstrated  that  heavy  dosages 
can  not  be  used  while  the  fruit  is  small  without  more  or  le^s  injury. 
Additional  evidence  has  been  secured  during  the  latter  part  of  this 
investigation  which  has  entirely  corroborated  the  earlier  experiments. 
Thus  it  may  be  staled  that  the  most  critical  period  for  conducting 
fumigation  is  between  the  time  the  fruit  sets  and  the  time  it  attains 
a  diameter  of  about  an  inch.  This  period  occurs  during  the  late 
spring  and  summer.  It  is  advised  not  to  fumigate  during  this  period, 
which  is  usually  from  April  to  August.  Although  in  some  cases  an 
orchard  may  be  treated  during  this  period  with  a  light  dosage  without 
injury,  yet  the  risk  is  too  great  to  justify  such  action.  Tt  is  better  to 
wait  until  the  regular  season  immediately  following  the  month  of 
July. 

FUMIGATING  LEMONS. 

Throughout  this  bulletin  the  recommendations  are  always  for 
citrus  trees,  which  include  the  orange,  the  grapefruit  (pomelo),  and 
the  lemon.  The  acreage  of  grapefruit  is  very  small.  Oranges  and 
lemons  occur  more  or  less  promiscuously  throughout  the  same  districts 
where  lemons  are  grown,  while  frequently  an  orchard  will  consist 
partly  of  each.  The  orange  and  grapefruit  are  about  equally  sus- 
ceptible to  injury  from  fumigation,  while  the  lemon  is  much  more 


66  HYDBOCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


resistant.  As  all  three  kinds  of  citrus  are  fumigated  at  the  same 
time,  regardless  of  kind,  it  is  necessary  to  base  advice  on  the  one  most 
susceptible  to  injury.  If  the  tenderest  kinds  escape  injury  the  more 
resist  ant  very  naturally  will  do  so.  Hence,  in  reality  the  recommen- 
dations in  this  bulletin  are  based  on  the  orange,  partly  because  of  its 
greater  acreage  but  mainly  because  any  recommendations  made  for 
it  will  apply  equally  to  the  grapefruit  or  lemon. 

The  lemon  blossoms  throughout  the  year  in  California,  so  that 
different  sized  fruit  may  be  found  at  all  times.  This  is  very  different 
from  the  orange,  which  has  one  regular  crop.  Lemons  usually  will 
escape  pitting  or  burning  under  conditions  which  might  seriously 
injure  oranges.  This  allows  a  wider  range  of  activity  in  fumigating 
lemon  trees.  The  work  can  be  continued  somewhat  later  in  the 
spring  and  commence  earlier  in  the  summer  than  with  the  orange. 
In  fumigating  a  section  containing  lemon  and  orange  trees  it  is  good 
policy,  where  convenient,  to  commence  on  the  lemons,  leaving  the 
oranges  until  a  later  period. 

EFFECTS  OF  FUMIGATION  ON  UNHEALTHY  TREES. 

Unhealthy  citrus  trees  are  found  universally.  Occasionally  a 
part  or  whole  of  an  orchard  is  composed. of  trees  weakened  by  lack  of 
such  essential  treatment  as  proper  cultivation,  fertilization,  or  irri- 
gation. Many  orchards  contain  trees  weakened  from  attacks  of  a 
gum  disease,  of  "gophers"  (ground  squirrels),  scale  insect  pests, 
and  numerous  other  causes  which  check  their  normal  development. 
These  unhealthy  trees  are  less  resistant  to  injury  from  fumigation 
than  perfectly  healthy  ones.  In  examining  results  in  an  orchard 
recently  fumigated  the  writer  has  noticed  frequently  that  the  fruit 
on  a  few  trees  that  had  been  weakened  by  disease  was  severely  pitted 
or  burned,  while  that  on  all  healthy  trees  was  uninjured.  A  heavy 
dropping  of  fruit  might  have  taken  place  in  the  unhealthy  trees,  while 
the  others  were  unaffected  in  any  way.  A  most  striking  example 
of  severe  injury  to  unhealthy  trees  was  seen  in  an  orchard  fumigated 
with  double  tents  (one  tent  over  the  other)  using  a  dosage  twice 
schedule  No.  1 .  Healthy  trees  in  some  cases  were  severely  burned 
back  at  the  top  for  about  a  foot,  accompanied  by  the  dropping  of 
some  leaves,  while  the  trees  weakened  by  gum  disease  usually  would 
be  burned  back  from  2  to  3  feet  and  drop  practically  all  their  leaves. 
Severe  injury  to  unhealthy  trees  has  been  seen  even  where  the 
three-fourths  schedule  was  used. 

Practical  fumigators  have  always  been  aware  of  the  susceptibility 
of  weakened  trees  to  injury  and  have  decreased  their  dosage  greatly 
in  1  reating  such  trees.  The  grower  should  not  complain  if  the  fruit 
and  leaves  on  their  unhealthy  trees  are  slightly  injured.  Such  fruit 
is  normally  of  the  inferior  grades,  while  the  damage  caused  by  the 


FUMIGATION  OF  CITRUS  TREES. 


67 


■bedding  of  leaves  is  more  apparent  than  real.  These  weakened 
leave-  normally  would  not  be  held  on  the  trees  much  longer.  The 
fumigation  merely  hastens  their  removal  and  is  usually  followed  by  a 
fresh  invigorated  growth  superior  in  all  respects  to  the  old. 

GREATER    SUSCEPTIBILITY    TO    INJURY    OF    SOME  VARIETIES 

THAN  OTHERS. 

The  lemon  tree  is  much  more  resistant  to  injury  from  fumigation 
than  the  orange  and  seldom  suffers  any  appreciable  damage  when 
treated  under  normal  conditions.  Some  varieties  of  oranges  are 
more  easily  injured  than  others.  Of  the  varieties  of  commercial 
importance  in  California  the  Navel  and  Valencia  are  the  least  sus- 
ceptible to  injury  from  the  gas  treatment.  The  seedling  is  almost 
equally  hardy.  Next  comes  the  Mediterranean  Sweet  while  the 
Homosassa  and  St.  Michael  can  seldom  be  treated  with  schedule 
No.  1  without  some  injury  resulting.  Fortunately  the  Navel  and 
Valencia  comprise  the  bulk  of  the  oranges  grown  in  this  State. 

THE  DISTRIBUTION  OF  GAS  WITHIN  A  TENT. 

Hydrocyanic-acid  gas,  being  lighter  than  air.  has  a  tendency  to 
rise  toward  (he  top  of  the  tent.  The  <<>luinn  of  gas  rises  straight  up 
from  the  generating  vessel  until  broken  up  by  coining  in  contact  with 
the  leaves  and  branches  of  the  tree.  The  greater  density  of  gas 
toward  the  top  of  a  tent  is  indicated  by  the  greater  amount  of  injured 
fruit  there  than  elsewhere.  Only  infrequently  is  fruit  at  the  bottom 
of  the  tree  pitted.  Dr.  Morrill  has  given  records  of  the  difference  in 
destruction  to  the  citrus  white  fly  at  different  heights  in  a  tree.1 
Similar  results  have  been  observed  against  the  scale  insects  in  Cal- 
ifornia. The  insects  at  the  top  of  a  tree  may  all  be  destroyed  while 
some  on  branches  close  to  the  ground  will  escape.  Hence,  in  the  ease 
of  the  purple  scale,  when  the  infestation  is  generally  toward  the  bot- 
tom of  the  tree  the  necessity  of  a  strong  gas  is  evident. 

FUMIGATION  FOR  PHYSIOLOGICAL  EFFECTS. 

There  are  people  in  California  who  believe  that  citrus  trees  should 
be  treated  with  hydrocyanic-acid  gas  whether  they  are  infested  with 
scale  or  not;  that  the  treatment  invigorates  the  tree,  producing  a 
heavier  crop  and  superior  fruit  than  would  otherwise  result. 

A  tree  infested  with  scale,  on  being  relieved  of  its  burden  responds 
to  the  treatment.  This  response  is  not  due  to  the  physiological  action 
of  the  gas  on  the  tree  itself  but  rather  to  the  destruction  of  the  large 
number  of  insects  which  have  been  constantly  sapping  the  plant 
juices.  The  removal  of  this  heavy  drain  allows  the  tree  to  resume 
its  normal  activity,  which  it  does  by  first  producing  invigorated 
growth. 


1  Bid.  76,  Bur.  Eut.,  U.  S.  Dept.  Agr.,  p.  51,  1908. 


68  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

During  this  investigation  many  acres  of  perfectly  healthy  trees  free 
of  insect  pests  have  been  treated  throughout  the  period  of  several 
months  preceding  blossoming  up  until  the  blossoms  fall.  No  effect 
producing  an  increase  or  perfection  of  the  coming  crop  of  fruit  has 
ever  resulted  from  these  efforts.  Although  the  acreage  treated  has 
not  been  sufficiently  great  to  justify  the  absolute  statement  that 
fumigation,  in  itself,  never  produces  a  greater  crop  of  fruit,  never- 
theless the  negative  results  in  all  the  orchards  treated  certainly  prove 
that  if  an  increased  crop  ever  results  it  is  of  infrequent  occurrence  and 
under  peculiar  conditions. 

EFFECTS  OF  METEOROLOGICAL  ELEMENTS  ON  FUMIGATION. 

MOISTURE. 

The  great  affinity  of  water  for  hydrocyanic-acid  gas  is  well  known. 
Writers  on  greenhouse  fumigation  contend  that  the  plants  should  be 
dry  when  treated,  else  injury  might  result.  It  has  been  the  universal 
opinion  among  California  fumigators  that  if  the  fumigation  treatment 
was  carried  on  while  the  trees  were  wet  injury  might  follow.  Gos- 
sard,1  and  more  recently  Morrill,2  assert  that  moisture  does  not  pro- 
duce injury  to  the  fruit  and  that  the  destruction  of  insects  is  as  great 
under  wet  as  under  dry  conditions.  Quaintance  3  has  corroborated 
these  results  of  Gossard  and  Morrill  where  the  work  is  done  on  moist 
fruit  (apples)  in  cold  storage. 

The  fumigation  work  which  has  been  performed  by  the  Govern- 
ment outfit  during  this  investigation  has  been  carried  on  in  all  sections 
of  southern  California,  at  all  times  of  the  year,  and  under  all  conditions 
of  weather — when  the  trees  were  entirely  dry,  when  the  trees  were 
wet,  so  wet  that  the  moisture  was  falling  off  in  drops,  and  when  it  was 
raining.  Mr.  G.  R.  Pilate,  a  temporary  assistant,  was  stationed  with 
a  practical  outfit  during  October,  November,  and  December,  1909, 
at  Rivera  and  Downey,  than  which  the  writer  believes  there  is  no 
generally  damper  citrus  section  in  California.  Careful  records  were 
kept  of  all  the  climatic  elements  which  might  affect  fumigation. 
These  records  show  that  almost  every  night  the  trees  became  thor- 
oughly moistened  before  the  work  was  discontinued.  In  addition  to 
this,  the  results  of  the  treatment  of  hundreds  of  acres  by  other  prac- 
tical outfits  have  been  followed  during  the  writer's  residence  in 
southern  California. 

From  all  this  experience  not  a  single  authentic  instance  has  been 
seen  in  which  burning  was  directly  attributable  to  absorption  of  gas 
by  the  moist  ure  on  the  fruit  or  leaves.  Thus  the  writer  has  felt  justi- 
fied in  concluding  that  the  presence  of  moisture  on  trees  can  be  ignored 

1  Bui.  <i7,  Fla.  Agr.  Exp.  Sta.,  pp.  647-G48,  1903. 

2  Bui.  7fi,  Bur.  Ent.,  U.  S.  Dept.  Agr.,  pp.  12-14, 1908. 
»  Bui.  84.  Bur.  Ent.,  U.  S.  Dept.  Agr.,  pp.  24,31,  1909. 


FUMIGATION  OF  CITRUS  TREES. 


69 


in  so  far  as  the  effect  of  its  direct  action  on  the  hydrocyanic-acid  gas 
treatment  is  concerned. 

There  are  other  reasons  of  indirect  and  largely  mechanical  nature 
on  account  of  which  it  is  necessary  to  consider  the  presence  of  mois- 
ture, for  ignorance  of  these  will  frequently  result  in  much  burned 
fruit.  (1)  When  touts  become  moist  they  become  heavier.  This 
renders  them  more  difficult  to  handle.  Much  fruit  is  torn  off,  while 
branches  and  limbs  are  frequently  broken.  (2)  The  damp  tents  col- 
lect much  dirt  and  as  they  are  pulled  over  the  trees  they  sometimes 
scrape  the  fruit  with  which  they  come  in  contact.  Such  abrased  fruit 
is  frequently  burned  by  the  action  of  the  gas.  (3)  As  the  trees 
become  damp  from  the  dew  or  fog,  whichever  it  may  be,  so  also  do 
the  tents  get  damp.  The  moisture  affects  the  fiber  of  the  cloth  so  t  hat 
it  becomes  tighter  and  retains  the  gas  better  than  when  dry.  Any 
person  of  any  considerable  experience  in  fumigation  knows  that  more 
gas  is  left  under  the  tents  when  they  are  pulled  off  on  a  damp  night 
than  on  a  dry  one;  therefore,  the  gas  remaining  in  the  tops  of  tall 
peaked  trees  is  much  more  concentrated  than  is  normally  the  case. 
This  intense  strength  of  gas  sometimes  causes  pitting,  especially  in 
the  case  of  some  varieties  least  resistant  to  hydrocyanic-acid  gas. 
Hence,  considering  the  disadvantages  resulting  from  wet  tents,  it  is 
evident  t hat  fumigation  should  not  be  carried  on  at  such  a  time. 
Fumigation  should  be  stopped  after  the  leaves  and  tents  become 
thoroughly  damp. 

The  author's  experience  has  been  that  the  presence  of  moisture  on 
trees  does  not  reduce  the  efficacy  of  hydrocyanic-acid  gas  against 
scale  insects.  Results  have  been  exactly  as  good  when  the  trees  were 
wet  as  when  dry,  and  observation  of  the  work  accomplished  by  com- 
mercial outfits  lias  corroborated  it. 

Experiments  by  Penny1  on  plants  in  a  closed  box  showed  that 
moisture  on  the  leaves  absorbed  the  gas.  This  would  make  it  appear 
that  moisture  on  the  foliage  of  orange  trees  absorbs  some  gas,  and 
undoubtedly  such  is  the  case.  Remembering  that  on  dry  nights  all 
the  gas  within  tents  of  the  character  ordinarily  used  has  escaped 
within  an  hour's  exposure  but  on  damp  nights  frequently  much  gas 
remains  when  the  tents  are  removed,  it  is  at  once  apparent  that  the 
retardation  of  gas  by  the  damp  cloth  easily  offsets  any  absorption 
which  may  have  taken  place. 

TEMPERATURE. 

Heat. — Heat  probably  is  the  factor  which  is  responsible  for  more 
injury  to  fruit  than  any  other  cause.  Throughout  the  experience  of 
commercial  fumigation  instances  of  very  severe  injury  have  occurred 
almost  every  year  in  some  one  of  the  different  citrus  fruit  producing 


»  12th  Ann.  Rep.  Del.  Agr.  Exp.  Sta.,  1900. 


70  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

regions.  The  cause  of  this  injury  has  for  the  most  part  been  an 
enigma,  but  it  has  been  known  to  occur  during  that  character  of 
weather  locally  spoken  of  as  "electric"  or  "Santa  Ana/'  which  is  in 
reality  a  dry  condition  of  the  atmosphere  caused  by  the  hot  dry 
winds  of  the  desert  sweeping  through  passes  in  the  mountains  onto 
the  lower  lands  adjacent  to  the  ocean.  The  nights  in  southern  Cali- 
fornia are  usually  cool,  but  during  these  disturbances  from  the  desert 
they  frequently  become  very  warm. 

Records  of  fumigation  when  the  temperature  was  about  65°  F. 
were  secured  during  the  autumn  of  1909.  A  part  of  an  orange  orchard 
was  treated  on  three  consecutive  nights  during  the  so-called  "elec- 
tric" weather,  when  temperature  ranged  from  66°  F.  to  77°  F.  Con- 
siderable burning  resulted.  Work  was  then  stopped  for  a  week  until 
the  nights  became  cooler,  when  the  rest  of  the  orchard  was  treated. 
No  burning  at  all  occurred.  The  injury  to  the  fruit  during  the  hot 
nights  consisted  of  real  burns  covering  much  of  the  fruit — not  small 
pits  which  are  the  usual  indications  of  fumigation  injury.  This  burn- 
ing was  exactly  of  the  same  character  as  was  produced  in  a  large 
orchard  at  Red  lands  fumigated  during  the  late  summer  of  1908  on 
similar  warm  nights.  Records  have  been  secured  of  burning  in  other 
orchards  treated  under  similar  weather  conditions. 

From  the  data  at  hand  it  appears  that  injury  from  fumigation  will 
take  place  at  high  temperatures.  Based  on  the  author's  experiments, 
it  is  advised  that  fumigation  be  stopped  at  a  temperature  above  65°  F. 
Although  in  some  instances  work  may  be  carried  on  at  a  slightly 
higher  temperature  with  impunity,  the  risk  of  injury  appears  to  be  too 
great. 

Cold. — During  December,  1908,  a  part  of  an  orange  grove  at  Rivera 
was  fumigated  by  a  private  outfit  which  kept  temperature  records 
during  its  work.  At  the  time  the  work  commenced  the  temperature 
was  above  40°  F.  No  injury  was  done  until  the  fourth  or  fifth  set, 
when  the  temperature  had  fallen  to  37°  F.  This  set  was  slightly 
burned.  The  next  set  was  badly  burned,  much  fruit  dropping. 
Unfortunately,  the  temperature  for  this  set  was  not  taken.  The  fol- 
lowing set,  which  was  the  last,  was  made  while  the  temperature  was 
at  32°  F.  The  tops  of  many  of  these  trees  were  severely  burned 
back,  while  all  the  fruit  on  some  of  them  dropped. 

On  December  3,  1909,  exact  records  of  burning  from  fumigation 
were  obtained.  Three  sets  were  made  on  this  particular  night — the 
first  while  the  temperature  reached  about  36°  F.  to  37°  F. ;  during  the 
second  it  dropped  from  36°  F.  to  32°  F.;  while. during  the  third  set 
the  temperature  reached  31°  F.  The  first  set  was  slightly  injured; 
the  second  was  se  verely  burned  and  much  fruit  dropped;  while  prac- 
tieallv  .ill  fruit  on  (he  third  set  dropped,  and  some  trees  were  so 
severely  burned  as  to  lose  most  of  their  leaves. 


FUMIGATION  OF  CITRUS  TREES. 


71 


With  these  records  as  a  basis,  the  writer  advises  that  fumigation  be 
stopped  at  38°  F.  or  below.  It  is  known  that  there  are  instances  in 
which  fumigation  can  be  carried  on  with  impunity  at  a  temperature 
lower  than  38°  F.;  in  fact,  there  are  records  of  fumigation  carried  on 
as  low  as  33°  F.  without  any  considerable  injury.  The  fact  remains, 
however,  that  there  are  authentic  cases  of  severe  injury  at  tempera- 
tures below  3s",  and  any  person  carrying  on  work  at  these  lower 
temperatures  runs  the  risk  of  inflicting  injury,  and  with  products  of 
the  eommereial  value  of  citrus  fruits  one  can  not  well  afford  to  assume 
such  risks. 

LIGHT. 

Coquillett,  in  the  course  of  Ids  early  investigations  in  the  use  of 
hydrocyanic-acid  gas,  determined  that  daylight  fumigation  was  more 
injurious  than  fumigation  at  night.  This  he  attributed  principally 
to  the  fact  thai  the  actinic  rays  of  light  decomposed  the  gas  into  other 
gases  of  a  more  injurious  nature.  Commercial  fumigation  work 
since  then  has  been  carried  on  exclusively  at  night.  Occasionally  a 
person  of  questioning  mind  has  attempted  some  experiments  hoping 
to  dispel  the  old  idea  of  daylight  injury,  but  current  information  has 
it  that  these  attempts  have  never  been  successful. 

Some  records  of  daylight  work  have  been  taken  during  this  investi- 
gation. In  one  instance  about  50  large  orange  and  lemon  trees  were 
treated  .  partly  while  cloudy  and  partly  in  the  sunshine.  These  trees 
had  the  whole  upper  half  burned  back— branches  as  well  as  leaves 
and  fruit.  It  was  the  severest  injury  to  citrus  trees  the  writer  has 
ever  seen. 

One  very  cloudy  afternoon  about  2.")  trees  (orange  and  lemon)  were 
l  u ungated  for  red  scale  with  a  three-fourths  schedule.  The  tempera- 
ture ranged  between  70°  F.  and  80°  F.  The  lemon  trees  were  only 
Blightly  affe  ted,  but  the  orange  trees  were  very  severely  damaged,  a 
large  part  of  their  tops  being  burned  back. 

The  first  row  of  trees  fumigated  at  night  and  the  last  in  the  morn- 
ing arc  frequently  more  or  less  injured.  The  cause  of  this  injury  in 
the  former  case  is  that  the  work  is  sometimes  commenced  before  the 
Mm  has  set  and  while  it  is  still  warm.  In  the  latter  case  the  sun  has 
come  up  before  the  tents  were  removed  from  the  last  row  of  trees. 
The  injury  under  such  conditions  is  most  apparent  from  fumigation 
cai  i  ied  on  in  the  late  summer  and  early  autumn  when  the  weather  is 
warm.  On  the  cool  days  late  in  autumn  the  treatment  can  be  and 
is  carried  on  with  impunity  at  a  degree  of  light  at  which  injury  would 
result  on  the  warm  days  earlier  in  the  season.  Judging  from  the 
experiments  and  many  observations  which  the  author  has  made 
respecting  the  effect  of  fumigation  during  the  daytime,  the  contention 
that  daylight  work  is  generally  unsafe  is  entirely  correct.  Although 
67330°— Bull.  90—12  G 


72 


HYDROCYANIC- ACID  GAS  FUMIGATION 


IN  CALIFORNIA. 


the  rays  of  light  appear  to  have  a  direct  effect  in  producing  injury, 
these  investigations  have  shown  that  this  injunr  is  intensified  by  the 
presence  of  heat,  and  the  writer  contends  that  burning  during  the 
daytime  is  directly  attributable  to  heat  as  well  as  to  the  light  itself. 

Fumigation  should  never  be  attempted  in  the  daytime,  even  on 
cloudy  days.  Carry  on  the  work  at  night  and  do  not  commence  the 
operations  until  the  sun  has  disappeared  and  the  shades  of  darkness 
are  approaching.  Remove  the  last  row  of  tents  in  the  morning  before 
the  sun  rises. 

WINDS. 

Fumigation  should  never  be  attempted  during  a  heavy  wind,  for 
two  reasons:  First,  the  gas  escapes  out  of  the  tent  so  that  poor  work 
results;  second,  injury  to  the  trees  might  result.  Dr.  Morrill  has 
called  attention1  to  the  variability  of  results  on  trees  fumigated  during 
a  stiff  breeze,  stating  that  on  different  parts  of  the  same  tree  he  found 
the  killing  to  vary  from  30  to  100  per  cent.1  Observations  made  dur- 
ing this  investigation  cover  instances  in  which  the  gas  was  driven 
from  tents  by  winds  in  a  very  few  minutes.  The  trees,  of  course, 
required  a  second  treatment.  The  burning  of  trees  fumigated  during 
winds  has  frequently  been  observed.  In  light  winds  the  injury 
appears  to  be  more  prevalent  on  that  side  of  the  tree  from  which  the 
wind  comes.  Heavy  winds  appear  to  produce  the  burning  fully  as 
much  on  the  opposite  side,  or  may  affect  the  entire  tree.  A  sudden 
change  in  temperature  accompanying  a  wind  appears  to  be  especially 
severe  in  its  results.  Undoubtedly  some  of  the  cases  of  severe  burn- 
ing during  " electric"  weather  are  due  partly  to  the  wind  as  well  as 
to  the  heat. 

The  author's  experience  has  led  to  the  conclusion  that  fumigation 
should  never  be  carried  on  during  a  windstorm.  As  soon  as  a  breeze 
arises  sufficiently  strong  to  "flap"  the  tents,  it  is  well  to  discontinue 
work  until  calmer  weather. 

INJURY  TO  SPRAYED  TREES. 

Distillate  oils  are  still  used  by  a  few  orchardists  in  combating  scale 
pests.  As  these  men  become  discouraged  with  the  oil  treatment  they 
adopt  fumigation.  In  this  connection  it  appears  desirable  to  state 
a  recent  experience  in  the  fumigation  of  trees  previously  sprayed  with 
an  oil  combination  spray. 

A  lemon  orchard  of  40  acres  was  sprayed  early  in  the  autumn  with 
a  combination  of  Bordeaux  mixture  and  distillate  oil.  Two  months 
later  about  25  acres  of  this  orchard  were  fumigated  partly  with  a  No. 
1  and  partly  with  a  three-fourths  schedule.    This  fumigation  con- 


1  Bui.  76,  Hur.  Eut.,  U.  S.  Dept.  Agr.,  p.  12,  1998. 


FUMIGATION  OF  C  ITRUS  TREES. 


73 


tinned  over  a  period  of  fully  two  weeks,  much  of  which  was  ideal 
fumigation  weather.  The  resultant  injury  was  very  severe,  being 
marked  chiefly  by  a  dropping  of  leaves.  In  many  1  rees  t  he  leaves  and 
fruit  were  also  burned.  The  old  leaves  on  the  tree  at  the  time  of 
spraying  especially  were  affected,  the  number  of  these  shed  being 
sometimes  so  great  as  to  form  a  thick  blanket  underneath  the  tree, 
entirely  covering  the  ground.  In  these  very  same  trees  the  young 
tender  growth  at  the  top  of  the  tree,  which  had  appeared  since  the 
spraying  and  which  normally  is  the  first  to  he  injured  by  the  gas 
treatment,  escaped  uninjured. 

The  trees  were  healthy  and  well  cared  for,  which,  coupled  with  the 
fact  that  only  the  sprayed  portion  of  the  trees  was  affected  and  not 
the  younger  and  tenderer  growth,  proves  that  the  cause  of  the  injury 
was  the  spraying.  It  is  well  known  that  distillate  oil  weakens  a  tree, 
and  possibly  the  unnatural  addition  of  Bordeaux  mixture  makes  this 
weakening  even  more  severe.  As  further  proof  of  this  situation, 
trees  in  a  neighboring  orchard  similarly  sprayed  at  about  the  same 
time  were  fumigated.  Injury  resulted.  [Jnsprayed  trees  under  the 
same  conditions  and  treated  with  the  same  dosages  at  the  same  time 

were  uninjured. 

These  results  show  that  it  is  unsafe  to  Fumigate  trees  which  have 
been  recently  treated  with  a  Bordeaux-distillate  emulsion.  Although 
it  does  not  also  prove  that  trees  recently  sprayed  with  distillate  alone 
would  be  injured,  it  would  seem  good  policy  not  to  attempt  such 
fumigation  until  proof  of  its  harmlessness  has  been  secured. 

THE  APPEARANCE  OF  FUMIGATED  TREES. 

Orange  trees  containing  young  growth  usually  will  have  the  tender 
tips  of  this  growth  burned  back  with  the  ordinary  fumigation  dosage. 
The  wilting  of  this  affected  portion  is  visible  the  following  day.  espe- 
cially if  sunshiny.  During  cloudy  weather  the  effects  are  not  marked 
until  fully  a  day  afterwards.    The  tender  growth  in  Lemons  is  burned 

back  even  more  severely  than  in  the  orange.  Where  the  new  growth 
in  the  tops  of  the  tree  is  very  long  it  may  he  affected  for  6  inches  or 
even  a  foot. 

Some  weakened  old  leaves  might  be  shed  a  few  days  following  the 
treatment.  Healthy  leaves  are  seldom  shed  and  seldom  burn,  unless 
some  abnormal  condition  is  present.  Even  in  such  conditions  it  is 
t  he  fruit  that  is  first  injured. 

The  burning  back  of  the  tender  growth  does  not  injure  the  tree  in 
any  way.  With  such  vigorous  plants  as  citrus  trees  all  indications  of 
injury  have  disappeared  within  a  few  weeks  following  the  treatment. 
Fumigators  and  many  growers  look  upon  the  burning  of  young  growth 
as  an  indication  that  the  proper  dosage  has  been  given  the  tree  for 
good  results.    Of  course  such  assumptions  are  correct  only  in  part. 


74 


HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


THE  PRESENCE  OF  OLD  SCALES  ON  FUMIGATED  TREES. 

Scales  do  not  fall  from  the  different  parts  of  a  tree  as  soon  as 
destroyed.  On  young  and  growing  fruit  they  are  easily  shed,  but  they 
may  cling  to  the  old  fruit,  leaves,  and  branches  of  trees  until  removed 
by  mechanical  means.  Dead  purple  scales  probably  cling  to  the 
different  parts  of  a  tree  much  more  generally  than  any  of  the  other 
common  citrus  pests.  Leaves,  branches,  or  old  fruit  severely  infested 
will  normally  remain  so  as  long  as  they  continue  to  be  a  part  of  the 
tree.  This  condition  leads  many  growers  to  condemn  a  treatment  as 
unsuccessful  because  on  examining  a  tree  long  after  the  operation 
scales  are  found  present.  The  mere  presence  of  scales  may  incorrectly 
indicate  to  them  that  they  have  not  been  destroyed. 

The  prevalence  with  which  successive  generations  of  scale  insects 
exist  simultaneously  on  citrus  trees  renders  it  impracticable  and  really 
impossible  to  draw  conclusions  in  exact  percentages  as  to  net  results 
of  experimental  work  whenever  the  results  fall  short  of  eradication. 
At  the  time  of  the  treatment  the  scale  on  some  leaves  might  all  be 
alive,  while  on  other  leaves  the  majority  may  be  dead,  and  with  all 
gradation  between  to  be  found  elsewhere. 

A  DEVICE  FOR  COVERING  FUMIGATION  GENERATORS. 

During  the  course  of  this  investigation  much  attention  has  been  given 
to  perfecting  a  device  for  attachment  to  the  top  of  the  commonly  used 
open-style  fumigation  generator  that  will  serve  to  interrupt  the  direct 
rise  of  the  hydrocyanic-acid  gas.  The  result  of  these  efforts,  in  which 
the  writer  was  greatly  aided  by  Mr.  Frederick  M askew,  is  shown  in 
figure  12.  The  device  itself  consists  of  a  copper  cover  of  such  size  as 
to  make  it  available  for  use  with  any  of  the  regular-pattern  generators 
now  employed  by  the  f  umigators  of  southern  California.  It  is  stamped 
in  a  concave  form  from  a  sheet  of  copper,  with  corrugations  to  per- 
mit the  escape  of  gas.  The  shape  is  such  as  to  conform  to  the  size  of 
the  opening  of  generators  of  different  capacities  and  also  to  direct  the 
course  of  the  escaping  gas  downward  and  distribute  it  uniformly 
through  the  lower  part  of  the  tent.  It  is  attached  to  the  generator 
by  hinges  of  stout  copper  wire  secured  by  a  key  bolt  passing 
through  the  handle.  The  cover  is  raised  by  a  slight  pressure  of  the 
thumb  on  a  projecting  piece  which  is  curved  in  such  a  manner  that 
the  cover  will  remain  in  an  upright  posit  ion  when  so  required.  When 
the  generator  is  emptied  of  its  contents,  the  cover  swings  clear  by  its 
own  weight.  A  glance  at  the  illustration  will  satisfy  the  practical 
fumigator  that  it  is  adapted  to  all  the  requirements  of  rapid  work  in 
the  dark,  while  its  use  has  demonstrated  that  it  is  simple,  strong,  and 
durable.  It  is  very  possible  that  if  the  copper  cover  were  lined  with 
a  thin  covering  of  lead  its  durability  would  be  increased.    A  common 


FUMIGATION  OF  CITRUS  TREES. 


75 


result  of  the  use  of  heavy  dosages  of  fine  fragments  of  cyanid  is  the 
burning  and  ultimate  dropping  of  many  of  the  leaves  directly  above 
the  generator  in  the  pathway  of  the  rapidly  rising  gas.  This  result  is 
usually  spoken  of  as  the  "chimney''  effect.  The  generator  cover 
eliminates  this  "chimney''  burning. 

A  second  and  highly  important  point  is  the  effect  of  open  generators 
on  the  tent.  The  outer  part,  or  skirt,  as  it  is  sometimes  called,  of 
fumigating  tents  is  constantly  being  perforated  with  small  holes,  even 
when  used  by  the  most  careful  of  workers.  This  effect  has  been 
noticed  to  some  extent  in  the  outfit  of  this  investigation,  which  is 
believed  to  have  been  as  carefully  handled  as  any  fumigation  outfit 
could  be.  These  holes  are 
known  to  be  acid  burns.  A 
few  simple  tests  have  demon- 
strated conclusively  that 
many  of  these1  acid  holes  are 
due  to  acid  carried  along 
with  the  escaping  gas  and 
reaching  that  part  of  the  tent 
nearest  the  generator.  By 
placing  large  pieces  of  canvas 
in  the  path  of  ga>  escaping 
from  open  generators  in 
which  dosages  similar  to 
those  often  used  in  held  work 
are  employed  it  was  found 
that  drops  of  acid  reached 
the  canvas  as  high  as  5  feet 
from  the  ground.  The  writer 
has  frequently  seen  generat- 
ing vessels  placed  not  more 
than  2  feet  inside  the  tent. 
At  such  a  distance  one  can 
readily  see  that  drops  of  acid  might  reach  the  tent  .  The  cover  described 
above  so  deflects  the  gas,  and  incidentally  such  acid  as  is  carried  with 
it,  that  the  drops  are  thrown  to  the  ground,  thus  saving  the  tents. 
The  decreased  cost  in  mending  of  tents  will  doubtless  pay  for  the  cost 
of  such  a  cover  device  several  times  over  in  a  season  of  fumigation. 

A  third  advantage,  which  has  not  as  yet  been  demonstrated,  but 
which  there  is  reason  to  believe  will  develop,  is  a  better  distribution 
of  gas  through  the  tent.  Heretofore  the  most  difficult  part  of  the  tree 
in  which  to  destroy  insects  has  been  the  lowrer  part.  This  is  also  the 
pail  of  the  tree  in  which  the  purple  scale  is  largely  to  be  found. 
With  the  open  generator  the  gas  rises  straight  up  in  a  narrow  column 
for  several  feet,  being  broken  up  and  distributed  through  the  top  of 
the  tree  first.    As  the  gas  is  lighter  than  air,  it  is  not  to  be  expected 


Fig.  12.— A  cover  device  attached  to  a  fumigation  generator. 
Corrugations  in  cover  allow  gas  to  escape.  (Author's 
illustration.) 


76  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

that  it  will  quickly  become  uniformly  distributed  throughout  the  bot- 
tom of  the  tent  even  if  at  any  time  it  becomes  as  concentrated  here  as 
at  the  top.  The  greater  burning  effect  and  better  killing  effect  in  the 
top  of  the  tree  would  tend  to  substantiate  this  assumption.  Field 
observations  in  fumigating  large  trees  show  that  the  gas  is  of  no  great 
strength  at  the  lower  part  of  the  tent  for  several  minutes  after  the 
charge  is  set  off.  With  this  new  cover  the  gas  is  broken  up  and  dis- 
tributed through  the  bottom  of  the  tent  first.  By  the  time  it  reaches 
the  top  it  is  pretty  generally  distributed  throughout  the  tent.  As 
the  bottom  of  the  tree  is  among  the  first  to  receive  the  full  benefit  of 
the  gas,  a  more  complete  killing  of  scale  at  the  bottom  of  the  tent  may 
be  expected  than  with  an  open  generator. 

THE  EFFECT  OF  CLIMATIC  CONDITIONS   ON  SCALE  INSECTS. 

Climatic  conditions  exercise  a  marked  effect  on  the  different  insects 
affecting  citrus  trees.  The  purple  scale  and  black  scale  thrive  best  in 
the  more  moist  country  adjacent  to  the  ocean.  The  red  scale  thrives 
well  in  the  drier  interior  regions  of  southern  California  as  well  as  near 
the  coast,  while  the  yellow  scale  is  more  of  a  heat -withstanding  form 
than  any  of  the  others.  This  is  demonstrated  by  its  prevalence  in 
citrus  trees  in  the  hot  interior  valleys  of  northern  California  where  the 
purple  scale  and  to  a  large  extent  the  black  scale  appear  unable  to 
survive. 

The  direct  effect  of  heat  on  scale  insects  may  be  evidenced  by  data 
on  the  black  scale  collected  during  the  summer  of  1907.  Com- 
mencing at  Pasadena,  which  is  at  the  opening  or  gateway  of  the  San 
Gabriel  Valley,  the  writer  proceeded  to  Duarte,  Pomona,  Ontario, 
Riverside,  Orange,  and  Santa  Ana,  CaL,  respectively.  The  San 
Gabriel  is  one  of  the  interior  valleys  and  Pasadena  is  situated  near 
that  end  which  opens  toward  the  ocean.  As  one  approaches  River- 
side from  Pasadena  the  climate  becomes  generally  hotter.  Orange 
and  Santa  Ana  are  nearer  the  ocean  and  much  cooler  than  any  of  the 
other  places  examined. 

It  might  be  mentioned  that  about  a  month  previous  to  this  special 
investigation  there  had  been  a  very  hot  spell  of  a  few  days'  duration. 
At  Pasadena  examination  showed  that  about  one-fourth  of  the  eggs 
under  the  old  scales  were  dried  up  and  brown,  this  condition  showing 
the  effects  of  heat.  At  Duarte  the  destruction  was  somewhat 
greater.  At  Pomona  and  Ontario,  which  were  much  hotter  even 
than  the  two  preceding  places,  more  than  three-fourths  of  the  eggs 
and  young  insects  were  dead.  At  Riverside,  where  the  heat  was 
most  intense,  a  very  small  percentage  of  healthy  eggs  or  live  insects 
was  found.  In  the  cooler  sections  of  Orange  and  Santa  Ana  very 
much  less  than  a  fourth  of  the  eggs  and  young  were  destroyed,  while 
insects  in  all  stages  of  development  were  in  evidence. 


FUMIGATION  OF  CITRUS  TREES. 


77 


The  orchards  in  Riverside  and  San  Bernardino  Counties  are  seldom 
as  severely  infested  as  elsewhere.  These  observations  explain  why 
scale-insects  are  less  destructive  in  these  two  counties  than  in  regions 
nearer  the  coast.  The  hot  weather  appears  to  be  almost  as  efficient 
as  some  insecticide  treatments. 

THE  EFFECT  OF   FUMIGATION  ON  LADYBIRDS   | COCCINELLIDiE) 
AND  SCUTELLISTA  CYANEA  MOTSCH. 

Several  writers  on  fumigation  have  mentioned  that  ladybirds  are 
less  easily  killed  by  hydrocyanic-acid  gas  than  the  scale-insects  of 
the  citrus.  In  order  to  prove  this  contention,  specimens  of  the  two 
common  ladybirds  (Cocclnelhi  californiai  Mann,  and  ///' ppodamia  cou- 
rt rgens  (luer.)  were  suspended  under  a  fumigation  tent  in  an  open 
cage  6  to  7  feet  above  the  ground,  while  others  were  placed  right  on 
the  ground.  Schedule  No.  1  was  used.  The  tent  was  left  on  the 
tree  one  hour.  The  temperature  was  60°  F.  The  insects  were 
examined  about  ten  minutes  after  the  tents  had  been  removed,  but 
exhibited  no  signs  of  life  whatever:  all  appeared  to  be  dead.  The 
following  morning  a  second  examination  showed  that  many  had 
revived  from  their  previous  stupefied  condition.  A  count  made  of 
those  suspended  from  ti  to  7  feet  above  the  ground  showed  that  out 
of  a  total  of  hi.  :5_\  or  just  50  per  cent,  were  killed.  Of  85  on  the 
ground  33,  or  about  39  per  cent,  were  destroyed.  Both  species 
appeared  equally  resistant  to  the  gas. 

It  thus  appears  that  the  insects  on  the  ground  survive  a  gas  treat- 
ment somewhat  more  readily  than  those  toward  the  top  of  a  tree. 
When  it  is  considered  that  the  strength  of  gas  was  as  great  as  is  ever 
used  in  commercial  work  against  the  common  scale  insects  of  citrus 
trees,  in  fact  BOmewhal  stronger  than  is  used  by  most  fumigators,  and 
also  i hat  in  regular  operations  many  of  the  ladybirds  after  becoming 
stupefied  fall  oil'  onto  the  ground,  where  they  are  less  affected  by  the 
gas.  it  would  seem  safe  to  presume  that  the  larger  number  of  these 
insects  on  a  tree  at  the  time  of  fumigation  will  survive  the  treatment. 

Scutellista  cyanea  Motsch.  is  a  small  hymenopterous  parasite  of  the 
black  scale  which  was  introduced  into  California  from  South  Africa 
several  years  ago.  The  larva'  feed  on  tin4  eggs  of  the  black  scale, 
usually  one  in  each  scale,  although  there  may  be  2,  3,  or  even  4 
present.  Frequently  as  high  as  from  50  to  75  per  cent,  or  even  more, 
of  the  black  scale  on  a  tree  have  been  seen  attacked  by  this  parasite. 
The  work  of  this  insect  is  remarkably  good,  yet  not  sufficiently  perfect 
to  allow  the  fruit  to  come  into  the  packing  house  in  a  clean  condition. 
This  necessitates  fumigation  as  though  the  parasites  were  not  present. 

This  is  not  meant  to  depreciate  the  usefulness  of  this  parasite,  even 
though  it  fails  to  keep  citrus  trees  entirely  free  of  the  black  scale. 
By  destroying,  as  it  does,  a  large  percentage  of  eggs  it  confers  a 


78  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

decided  benefit.  The  prevalence  of  the  black  scale  on  many  other 
hosts  offers  a  wide  range  of  activity  for  the  parasite,  and  it  is  on  these 
noncitrus  plants  that  our  little  friend  does  some  of  its  very  best  work. 
If  it  did  nothing  else,  its  work  against  the  black  scale  on  the  pepper 
tree  (ScMnus  molle)  makes  it  especially  worthy  of  praise,  and  the 
question  has  frequently  come  up  whether  or  not  fumigation  destroys 
the  Scutellista.  Numerous  observations  and  experiments  respecting 
this  point  have  been  made  during  this  investigation  indicating  that 
fumigation  destroys  many  of  the  Scutellista  in  its  adult  and  pupal 
stages.  The  majority  of  the  parasites,  however,  are  unaffected,  even 
when  schedule  No.  1  is  used.  Parasitized  scales  have  been  removed 
the  day  following  such  treatment  and  placed  in  vials,  with  the  result 
that  some  adults  would  immediately  issue  and  others  continue  to 
issue  for  many  days  afterwards.  The  adults  of  Scutellista  in  the  open 
are  destroyed  by  a  weak  dosage.  The  reason  they  escape  beneath 
the  scales  is  that  such  parasitized  scales  are  tightly  sealed  to  the 
leaf  or  branch,  apparently  by  some  secretion  produced  by  the  larva?, 
and  the  gas  does  not  penetrate  such  scales  as  easily  as  it  does  those 
nonparasitized. 

One  of  the  greatest  benefits  of  the  Scutellista  is  its  work  in  trees 
which  have  been  fumigated.  The  eggs  of  the  black  scale  to  a  large 
extent  survive  the  gas  treatment.  This  leaves  abundant  oppor- 
tunity for  a  future  infestation  on  trees  treated  when  eggs  are  present. 
If  Scutellista  occurs  in  the  orchard,  these  undestroyed  eggs  are 
devoured,  thus  completing  a  treatment  for  which  fumigation  itself 
is  only  partially  successful. 

THE  COST  OF  FUMIGATION. 

The  cost  of  fumigating  an  orchard  depends  principally  on  the  size 
of  the  trees  and  the  dosage-rate  used.  The  average  California  citrus 
orchard  requires  an  average  expenditure  of  from  $25  to  $40  per  acre 
for  one  fumigation  treatment.  Large  seedling  trees  are  much  more 
expensive,  while  young  trees  cost  considerably  less. 

The  directors  of  fumigating  outfits  base  their  estimates  on  two 
distinct  considerations:  The  chemicals  and  covering  the  trees.  Con- 
tract fumigators  usually  furnish  the  cyanid  at  30  cents  per  pound, 
which  also  includes  the  sulphuric  acid  necessary  for  generation  of  the 
gas.  The  price  of  covering  trees  varies  with  their  size,  number, 
location,  topography  of  land,  etc.  The  fumigator  will  charge  more 
per  tree  where  the  orchard  consists  of  a  half  acre  than  if  it  has  50 
acres.  Trees  that  require  a  45-foot  tent  usually  will  cost  more  to 
cover  than  those  requiring  a  36-foot  tent.  The  average  price  of 
covering  in  commercial  work  where  nothing  larger  than  45-foot  tents 
is  used  is  from  10  to  12  cents  per  tree.  Large  seedling  trees  whose 
covering  requires  derricks  may  cost  from  40  to  50  cents,  or  even  more, 


FUMIGATION   OF   CITRUS  TREES. 


79 


per  tree.  After  completing  the  treatment  of  an  orchard,  knowledge 
of  the  number  of  trees  covered  and  the  amount  of  cyanid  used  fur- 
nishes immediate  means  of  calculating  the  cost. 

The  estimates  of  association,  county,  and  private  work  will  vary 
somewhat  from  the  above  figures,  for  in  these  cases  the  work  is  sup- 
posed to  be  performed  at  actual  cost.  The  following  figures  enter 
here  and  are  the  average  of  field  conditions.  Cyanid  is  purchased  at 
practically  25  cents  per  pound  in  large  quantities  (tons)  and  25$  cents 
in  smaller  lots.  Sulphuric  acid  costs  about  1J  cents  per  pound. 
Five  men  are  required  to  run  an  outfit.  The  foreman  receives  about 
50  cents  per  hour,  while  the  other  4  men  receive  about  35  cents  each 
per  hour.  This  makes  a  total  cost  of  about  $1.90  per  hour  for  labor. 
By  adding  to  the  cost  of  labor  the  cost  of  cyanid  and  acid,  as  well  as 
allowing  a  certain  amount  for  transporting  the  chemicals  to  the  held, 
and  including  the  cost  of,  as  well  as  wear  and  tear  on  the  tents  and 
other  equipment  necessary  in  fumigation  work,  we  have  a  basis  for 
estimating  the  real  cost  of  the  operation. 

Most  trees  fumigated  require  between  -r>  and  IS  ounces  of  cyanid. 
An  average  dosage  would  be  about  10  ounces.  A  supply  cart  (pp. 
29-93)  can  be  equipped  complete  for  about  $35. 

Generating  pots  cost  as  follows:  One  and  one-half  gallon,  35  cents; 
J-gallon,  45  cents;  3-gallon,  65  cents. 

Below  are  given  the  prices  of  different  sized  octagonal  sheet  tents 
ready  for  use,  as  furnished  by  one  of  the  largest  dealers  in  fumigating 
tents  in  southern  California.  These  prices  are  based  on  the  assump- 
tion that  an  entire  outfit  of  30  tents  is  to  be  purchased.  If  only  a 
single  tent  is  purchased,  the  cost  will  be  slightly  greater  than  these 
figures.  Fluctuations  in  the  cotton  market  will  cause  a  variation  in 
the  price  of  tents. 


Size. 

230  tr- 
ounce) 
special 
drilD 

200  (8- 
ounce) 
special 

8-ounce 
special 
Army 
duck. 

$6.4*5 
12. 92 
19.95 
28.50 
36. 10 
43.70 
45.60 
50.35 
62.  70 
69.35 
91.20 
114. 76 

$6. 97 
13.94 
21.53 
30. 75 
38. 75 
47.15 
49.20 
54.32 
67.65 
74.82 
98.40 
123.82 

$6.12 
12. 24 
18.90 
27.00 
34.20 
41.40 
43.20 
47.70 
59.40 
65.70 
86.40 
108.72 

24-foot  

30-foot  

36-foot  

41-foot  

43-foot  

48-foot  

52-foot  

55-foot  

04-foot  

72-foot  

1  The  7  and  8  ounce  special  drills  are  those  recommended  by  this  investigation  (p.  12).  The  inferior  grades 
of  drill  ordinarily  employed  are  about  20  per  cent  cheaper  than  the  7-ounce  special. 


The  cost  of  thirty  45-foot  tents  of  special  7-ounce  drill  together  with 
the  other  equipment  necessary  to  complete  the  outfit  will  approxi- 
mate SI, 400. 


80  H  YDROC  YANTC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

GENERAL  CAUTIONS. 

Hydrocyanic-acid  gas  is  one  of  the  most  deadly  of  gases,  so  that 
considerable  care  is  necessary  in  its  use.  Such  exaggerated  cautions 
have  been  written,  in  view  of  its  poisonous  properties,  that  the  public 
at  large  have  come  to  believe  that  a  single  whiff  of  this  gas  will 
produce  the  immediate  death  of  an  individual.  This  erroneous  idea 
should  be  corrected.  A  whiff  of  the  gas  will  not  cause  immediate 
death  ;  neither  will  two  or  three  whiffs.  If  subjected  to  a  strong  gas 
for  a  minute  or  two,  undoubtedly  a  person  would  be  overcome.  The 
writer  has  never  yet  had  a  record  of  a  person  killed  by  hydrocyanic- 
acid  gas  while  fumigating.  In  California,  men  work  around  tents 
every  night  for  weeks  at  a  time  without  any  ill  effects.  During  these 
operations  they  are  breathing  the  gas  in  a  diluted  form  much  of  the 
time.  Repeatedly  field  men  are  seen  sitting,  either  to  rest  or  eat 
their  lunch,  on  the  edge  of  a  tent  covering  a  tree  which  had  been 
dosed  a  few  minutes  previously.  The  writer  has  personally  stood 
within  a  foot  of  a  generator  for  an  hour  at  a  time,  taking  tempera- 
tures of  the  escaping  gas  as  different  dosages  were  tried  out,  some 
of  which  would  be  in  excess  of  a  pound.  Frequently  the  wind  would 
waft  the  fumes  into  his  face.  Outside  of  an  occasional  dizziness  and 
headache,  no  serious  results  were  experienced.  Scores  of  similar 
cases  could  be  cited. 

These  experiences  have  been  mentioned,  not  with  the  idea  of 
tempting  people  to  be  careless  in  the  use  of  this  gas,  but  merely  to 
correct  the  erroneous  conception  that  a  whiff  of  the  gas  will  cause 
instant  death.  This  gas  is  most  dangerous,  and  the  writer  has  seen 
men  who  were  subjected  to  a  great  strength  of  it  for  several  minutes 
at  a  time  overcome  by  its  effects,  although  they  revived  later.  If  the 
proper  precautions  are  taken  the  careful  operator  will  run  no  risk 
whatever.  Place  the  charge  in  the  generating  vessel  with  extended 
arm  so  that  the  head  of  the  operator  is  away  from  the  escaping  gas. 
Being  lighter  than  air,  the  gas  rises  straight  up  in  a  narrow  column 
until  several  feet  above  the  ground.  As  soon  as  the  dosage  has  been 
set  off,  quickly  leave  the  tent  or  room,  whichever  it  may  be.  If  this 
procedure  is  followed  there  is  no  danger  of  ill  effects  to  the  operator. 


FUMIGATION  OF  CITRUS  TREES. 


81 


LIST  OF  THE  WRITER'S  PUBLISHED  ARTICLES  AND  ADDRESSES 
ON  THE  FUMIGATION  INVESTIGATION  IN  CALIFORNIA. 

The  following  list  includes  the  addresses  and  reports  made,  and 
the  papers  issued  by  the  writer  during  this  investigation: 

190S.    Investigations  of  the  use  of  hydrocyanic-acid  gas  in  fumigating  citrus  trees. 

Lecture  delivered  at  the  Thirty-fourth  Convention  of  the  California  State 
Fruit  Growers,  Riverside,  Cal.,  April  30,  1908.  <Printed  in  the  proceed- 
ings of  this  convention,  pp.  103-1  1 1 . 

Fumigation  investigations.  Talk  given  before  meeting  of  the  Whittier 
Farmers'  dub,  Whittier.  Cal.,  August  12,  1908. 

Fumigation  investigations  in  California.  Lecture  delivered  before  the  Hor- 
ticultural Commission  of  Los  Angeles  County,  fruit  growers,  and  fumigators, 
courthouse,  Los  Angeles.  CaL,  August  15,  1908.  ^California  Cultivator, 
August  20,  1908. 

Device  lor  covering  fumigation  generators.    In  cooperation  with  Frederick 

Maskew.    <California  Cultivator.  October  I,  1908. 
An  improved  system  of  fumigation.    Talk  given  at  the  annual  picnic  of  the 

Pomona  Farmers'  Club,  Pomona,  CaL,  October 5,  IM08. 
Fumigation.    Talk  given  at  Farmers'  Institute.  Ontario,  Cal.,  October  28, 

1908. 

The  use  of  water  in  fumigation.    <Los  Angeles  Times  Sunday  Magazine, 

p.  902,  November  8,  1908. 
The  latest  in  fumigation.    Paper  presented  at  the  Thirt y-tit'th  Convention  of 

the  California  State  Fruit  Growers,  Sacramento,  CaL,  December  2,  190s. 

<  Printed  in  Third  Biennial  Report  of  the  Commissioner  of  Horticulture  of 

the  State  of  California  for  1907-8,  pp.  99-112. 

1909.  Fumigation  investigations  in  California.    <Bul.  79,  Bur.  Ent.,  I'.  S.  Dept. 

Agr 

Marking  of  tents  for  practical  fumigation.  ^California  Cultivator,  August  12, 
1909. 

Fumigation  dosage  for  the  pests  of  citrus  trees.  ^California  Cultivator,  Sep- 
tember 16,  1909. 

1910.  Fumigation  in  California.    Lecture  delivered  before  horticultural  officers, 

fruit  growers,  and  fumigators,  courthouse,  Los  Angeles,  Cal.,  February  12, 
l!HC  ^California  Cultivator,  February  24  and  March  3.  Also  issued  in 
pamphlet  form  by  the  San  Antonio  Fruit  Exchange,  Pomona,  Cal. 

Value  of  sodium  cyanide  for  fumigation  purposes.  <Journ.  Econ.  Ent., 
vol.  3,  no.  1,  pp.  85-88. 

Morrill  system — a  reply.    ^California  Cultivator,  April  7,  1910. 


U.  S.  D.  A.,  B.  E.  Bui.  90.  Part  II. 


Issued  May  10.  1911. 


BYDROCYANIOACID  GAS  FUMIGATION  [N  CALIFORNIA. 


THE  VALUE  OF  SODIUM  CYANID  FOR  FUMIGATION 

PURPOSES. 

By  R.  S.  Wooi.im.  M.  S.  A.. 
SjHci'iJ  Field  Agent,  Bureau  of  Entomology. 

INTRODUCTION. 

The  present  treatise  comprises  the  results  secured  with  sodium 
cyanid  during  the  investigation  of  the  use  of  hydrocyanic-acid  gas 
for  the  fumigation  of  citrus  trees  in  California  and  is  supplementary 
to  the  extended  report  on  the  use  of  potassium  cyanid.  The  results 
with  the  sodium  salt  are  issued  as  a  separate  part  of  the  report  in 
order  to  avoid  confusion  between  its  use  and  that  of  the  universally 
used  pot assium  cyanid. 

Numerous  cyanids  which  will  furnish  hydrocyanic-acid  gas  are 
known  to  chemists,  hut  only  two,  those  of  potassium  and  sodium, 
possess  all  the  requirements  essential  for  commercial  work.  Potas- 
sium cyanid  was  the  chemical  first  used  in  California  in  the  genera- 
tion of  hydrocyanic-acid  gas,  and  has  continued  to  he  employed  for 
this  purpose  until  to-day  it  is  popularly  considered,  both  in  this 
country  and  abroad,  as  the  only  source  from  which  gas  can  be  econom- 
ically made.  The  grade  of  potassium  cyanid  furnished  during  recent 
years  for  citrus  fumigation  in  southern  California  lias  generally  been 
of  such  a  high  degree  of  purity  that  no  special  effort  has  ever  been 
made  to  find  a  substitute. 

With  the  exception  of  fumigation,  sodium  cyanid  is  used  more 
extensively  for  general  commercial  purposes  than  potassium  cyanid. 
C.  P.  Lounsbury,  government  entomologist  of  Cape  Colony,  was  the 
first  to  call  particular  attention  in  literature  to  sodium  cyanid  for 
fumigation  when,  in  1905,  he  wrote: 

It  is  possible  that  within  a  few  years  cyanid  of  soda  will  be  used,  instead  of  cyanid  of 
potash,  as  the  source  of  the  gas.  The  soda  compound  gives  off  more  gas  from  a  given 
weight  and  costs  no  more;  hence  by  its  use  some  saving  may  be  made  in  both  original 
cost  and  transportation  c  harges.  But  the  reaction  with  acid  appears  to  be  more  violent 
than  with  potash  salt,  which  is  a  disadvantage  owing  to  a  greater  risk  of  the  covers 
getting  burnt,  and  its  adoption  would  necessitate  an  entire  revision  of  our  fumigation 

S3 


84  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


tables.  The  quantities  of  acid  and  water  for  a  tree  of  any  given  size  would  remain 
the  same,  but  the  weight  of  cyanid  for  use  with  any  given  measure  of  acid  would  have 
to  be  reduced  one-fourth.1 

This  last  statement  of  Prof.  Lounsbury  assumes  that  the  reaction 
would  require  the  same  proportion  of  chemicals  as  with  potassium 
cyanid,  an  assumption  which  the  earlier  chemical  work  of  this  inves- 
tigation shows  to  be  without  any  foundation. 

The  potassium  cyanid  used  in  California  for  fumigation  is  manu- 
factured in  Germany,  and  is  popularly  styled  "German"  cyanid. 
This  is  in  distinction  to  a  grade  of  cyanid  made  in  this  country, 
termed  "  American"  cyanid,  which  has  had  an  occasional  and  limited 
usage  in  California  for  a  number  of  years.  This  so-called  "  American ' ' 
cyanid  has  been  popularly  believed  to  be  the  very  same  kind  as  the 
imported  potassium  cyanid.  wSuch  a  conception  has  been  misleading, 
for  chemical  analyses  have  shown  it  to  be  not  potassium  cyanid  but  a 
cyanid  of  sodium  equivalent  to  a  potassium  cyanid  of  98  to  100  per 
cent  purity. 

This  98  to  100  per  cent  sodium  cyanid  was  found  by  fumigators  to 
be  inferior  to  the  regular  potassium  salt  ordinarily  employed.  It  was 
of  very  hard  composition  and  generated  so  slowly  that  large  propor- 
tions of  acid  were  commonly  used  in  an  attempt  to  hasten  the  reaction. 
Some  fumigators  used  large  quantities  of  water,  believing  this  was 
necessary  in  order  to  secure  the  best  results.  Tests  performed  by  the 
writer  showed  that  frequently  it  would  require  10  to  15  minutes  before 
all  the  available  gas  was  expelled  from  a  generation  of  this  cyanid. 
Furthermore,  the  writer  was  informed  by  the  president  of  a  firm  which 
supplies  most  of  the  cyanid  used  in  California,  that  extensive  chemical 
experiments  carried  on  in  its  laboratory  indicated  that  this  sodium 
cyanid  produced  a  less  satisfactory  generation  than  did  potassium 
cyanid,  and  from  these  considerations  he  believed  its  use  would  never 
be  economical.  The  work  of  the  firm  also  indicated  that  large  pro- 
portions of  water  were  necessary  for  a  satisfactory  and  rapid  reaction. 

Under  the  belief  that  sodium  cyanid  had  not  received  sufficiently 
exhaustive  tests,  the  writer  ignored  all  local  unfavorable  evidence, 
and,  in  December,  1908,  outlined  in  detail  a  broad  series  of  chemical 
experiments  to  secure  reliable  data,  with  reference  to  this  salt,  on  the 
various  factors  which  might  enter  into  hydrocyanic-acid  generation. 
This  outline  was  submitted  to  the  Bureau  of  Chemistry  of  this  depart- 
ment for  execution.  Mi'.  C.  C.  McDonnell,  under  the  direction  of  Dr. 
J.  K.  Haywood,  Chief  of  the  Miscellaneous  Division,  made  a  very 
careful  and  elaborate  series  of  detenninations,  and  in  August,  1909, the 
results  of  these  tests  were  submitted  to  the  writer  in  a  carefully  pre- 
wired manuscript,  w  hich  forms  Part  J II  of  this  bulletin.  These 
results  are  used  in  part  in  the  preparation  of  the  present  paper. 


>  Office  Of  Government  Entomologist,  Department  of  Agriculture,  Cape  o\  Good  Hope.  "Cyanid  Gas 
Remedy  for  Scale  Insects."    July,  1905. 


SODIUM  CYANID  FOR  FUMIGATION  PURPOSES. 


85 


STRENGTH  OF  SODIUM  CYANID  EXPRESSED  IN  TERMS  OF 
POTASSIUM  CYANID. 

The  strength  of  sodium  cyanic!  is  commonly  reckoned  in  terms  of 
potassium  cvanid.  Chemically  pure  potassium  cvanid  is  100  per  cent 
pure,  while  chemically  pure  sodium  cvanid  is  commonly  spoken  of  as 
133  per  cent  pure.  The  explanation  for  this  is  that  pure  sodium  cva- 
nid contains,  for  the  same  weight,  practically  one-third  more  cyanogen 
(available  hydrocyanic-acid  gas)  than  potassium  cvanid  and  this 
greater  strength  is  expressed  in  terms  of  the  potassium  salt  for  con- 
venience sake.  Thus  we  have  133  per  cent  sodium  cvanid,  meaning 
that,  when  absolutely  pure,  it  contains  33  per  cent  more  cyanogen 
(available  hydrocyanic-acid  gas)  than  a  pure  potassium  cvanid.  It 
is  evident,  therefore,  that  if  1  pound  of  pure  sodium  cvanid  can  be 
purchased  cheaply  as  1  pound  of  pure  potassium  cvanid,  there  will 
result  a  great  economy  by  the  use  of  the  former  provided  the  genera- 
tion of  the  iras  be  equally  perfect  for  each.  The  price  is,  however, 
usually  based  od  the  percentage  of  cyanogen  present  and  the  saving 
in  cost  may  be  thus  offset  to  a  large  extent. 

PROPORTION  OF  CHEMICALS. 

A  large  number  of  experiments  was  carried  on  with  a  high-grade 
sodium  cvanid,  with  the  idea  of  determining  the  best  proportion  of 
chemicals  for  use.  As  a  result  of  these  tests  Mr.  McDonnell  recom- 
mended a  3-4-6  formula:  3  ounces  (avoirdupois)  of  cvanid,  4  fluid 
ounces  of  sulphuric  acid,  and  G  ounces  of  water.  Reduced  to  its  lowest 
units  for  rapid  work  in  the  field,  the  writer  lias  used  1  §  fluid  ounces  of 
acid  and  2  ounces  of  water  to  each  ounce  (avoirdupois)  of  cyanid  of 
sodium.    This  1—1  £-2  formula  is  recommended. 

The  1-1 £-2  formula  with  a  high-grade  sodium  cyanid  produces 
exactly  as  satisfactory  a  generation  in  the  laboratory  as  can  be 
obtained  from  a  high-grade  potassium  cyanid,  using  the  proportion 
of  chemicals  adapted  for  the  best  generation  of  gas  with  the  latter. 
This  is  exhibited  by  a  comparison  of  the  generation  from  two  high- 
grade4  commercial  products,  as  shown  in  the  following  table: 


Kind  of  cyanid. 

Proportion  of— 

Purity  of 
cyanid. 

Pet  cent 
of  total 

Bras 

evolved. 

Cyanid. 

Acid. 

Water. 

! 

1 

1* 

2 
2 

Per  cent. 
98 
124 

93.88 
94. 32 

Although  these  laboratory  determinations  exhibited  that  the  yield 
of  gas,  or  reaction,  with  a  high-grade  sodium  cvanid  is  as  satisfactory 
as  with  a  potassium  cyanid,  the  writer  did  not  feel  justified  in  recom- 
mending its  use  until  these  results  could  be  confirmed  by  actual  field 
experience. 


86 


HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


FIELD  TESTS. 

In  August,  1909,  1 J  acres  of  orange  trees  at  Whittier,  Cal.,  severely 
infested  with  the  purple  scale  (Lepidosa  plies  becJcii  Newm.),  which 
was  largely  in  the  egg  stage — the  most  difficult  one  in  which  to 
destroy  it — were  fumigated  with  a  124  per  cent  sodium  cyanid,  using 
the  1—1  ^—2  formula.  The  length  of  exposure  was  one  hour.  Three 
strengths  of  gas  were  used,  calculated  from  the  cyanogen  present  in 
the  cyanid  as  equivalent  to  1,  1^,  and  1J,  of  the  strength  of  regular 
schedule  Xo.  1  of  potassium  cyanid,  as  given  in  this  Bulletin.  The 
writer  says  strengths  equivalent  to  these  different  schedules  of  potas- 
sium, because  it  is  known  from  previous  discussion  that  an  ounce  of 
high-grade  sodium  cyanid  will  produce  considerably  more  gas  than  an 
ounce  of  high-grade  potassium  cyanid  owing  to  its  higher  cyanogen 
content.  In  the  124  per  cent  sodium  cyanid  the  advantage  in  favor 
of  that  product  is  practically  one-fourth.  It  follows  that  if  a  high- 
grade  sodium  cyanid  is  used  with  dosages  of  the  same  size  as  three- 
fourths  of  schedule  No.  1  of  potassium  cyanid  practically  the  same 
strength  of  gas  will  be  given  off  as  with  potassium  cyanid  at  the 
dosages  of  full  schedule  No.  1  of  potassium  cyanid. 

In  examining  the  results  where  the  equivalent  of  schedule  Xo.  1  of 
potassium  cyanid  was  used,  only  4  live  insects,  2  with  healthy  eggs, 
were  found,  these  being  distributed  between  3  different  trees.  Xo 
live  insects  or  eggs  were  found  on  the  trees  treated  with  dosages 
equivalent  to  1\  and  1J  of  schedule  No.  1  of  potassium  cyanid. 

Experiments  against  the  purple  scale  where  potassium  cyanid  was 
used  have  shown  that  it  required  a  lj  schedule  of  that  chemical  to 
produce  practically  the  same  results  as  have  been  secured  in  this 
experiment  with  sodium  cyanid  at  a  strength  equivalent  to  No.  1 
schedule  of  the  potassium  cyanid.  With  this  high-grade  sodium  com- 
pound the  equivalent  of  a  1 J  schedule  of  potassium  cyanid  eradicates 
the  purple  scale,  whereas  with  potassium  a  H  schedule  has  been 
required  to  accomplish  this  result.  Hence  this  held  experiment 
would  indicate  that  under  the  crude  conditions  of  orchard  work 
sodium  cyanid  produces  not  only  as  good  a  generation  as  the  potas- 
sium but  a  decidedly  superior  one. 

In  a  second  experiment  an  acre  of  orange  trees  very  severely 
infested  with  purple  scale  was  fumigated^  partly  witli  a  high-grade 
-odium  cyanid  and  partly  with  the  regular  potassium  cyanid.  The 
tents  used  were  of  a  new  character  of  material  of  very  tight  texture. 
The  sodium  cyanid  produced  exactly  as  satisfactory  results  in  this 
experiment  as  the  potassium  cyanid. 

During  November,  1909,  a  4-acre  block  of  orange  and  lemon  trees 
severely  infested  with  the  purple  scale  was  fumigated  witli  sodium 
cyanid  by  an  outfit  of  the  Whittier  Citrus  Association,  using  the 


SODIUM  CYAN  ID  FOR  FUMIGATION  PURPOSES. 


87 


l-li-2  formula.  The  dosage  strength  was  equivalent  to  that  of 
schedule  No.  1  of  potassium  cyanid.  The  operation  was  carried  out 
under  the  guidance  of  the  foreman  of  the  crew  exactly  as  work  is 
done  by  any  commercial  outfit.  Several  months  later  an  examina- 
tion was  made  of  a  large  number  of  trees  located  promiscuously 
throughout  the  orchard.  Live  purple  scales  were  seen  on  only  one  of 
the  trees  examined,  and  on  this  much  of  the  fruit  was  infested  at  the 
time  of  fumigation.  This  is  the  most  successful  work  the  writer  has 
ever  seen  done  by  a  practical  outfit  with  gas  of  a  strength  equivalent 
to  schedule  No.  1  of  potassium  cyanid. 

Several  hundred  pounds  of  this  high-grade  sodium  cyanid  were  used 
by  another  practical  outfit  in  fumigating  orange  and  lemon  trees. 
These  fumigators  were  as  satisfied  with  this  cyanid  as  with  the  regu- 
lar potassium  cyanid. 

From  the  results  of  field  work  in  this  investigation  it  has  been  found 
that  the  use  of  a  high-grade,  or  almost  chemically  pure,  sodium  cyanid 
produced  exactly  as  perfect  a  generation  of  gas  in  all  cases  as  the  use 
of  a  similar  grade  of  potassium  cyanid  ;  in  the  majority  of  cases  where 
used  the  generation  was  apparently  superior  to  that  from  a  potassium 
cyanid. 

ACTION  OF  SODIUM  CHLORID. 

The  above  recommendations  are  for  a  high-grade  sodium  cyanid  — 
one  almost  133  per  cent  pure.  Experience  in  California  witli  a  98 
to  100  percent  sodium  cyanid  has  proved  unsatisfactory.  The  rea- 
son is  given  herewith.  Chemical  analyses  have  shown  that  practi- 
cally nil  commercial  potassium  and  sodium  cyanids  contain  more  or 
less  common  salt,  which  is  technically  spoken  of  as  sodium  chlorid. 

Newell,1  in  1905,  pointed  out  that  sodium  chlorid,  when  present  in 
the  reaction  producing  hydrocyanic-acid  gas,  causes  a  secondary 
reaction  which  liberates  an  acid  called  hydrochloric  acid,  and  that 
this  liberated  hydrochloric  acid  immediately  attacks  the  hydrocyanic- 
acid  ga^,  decomposing  it  to  a  great  extent.  In  order  to  ascertain  more 
thoroughly  the  status  of  this  salt  Mr.  McDonnell  carried  on  a  large 
amount  of  experimental  work.  These  results  not  only  showed  con- 
clusively that  the  presence  of  sodium  chlorid  results  in  a  decom- 
position of  the  hydrocyanic-acid  gas,  but  also  that  if  a  sufficiently 
large  percentage  of  sodium  chlorid  is  present  the  decomposition  will 
be  so  great  as  to  result  in  little  if  any  hydrocyanic-acid  gas.  The 
conclusion  to  be  drawn  from  these  experiments  is  that  the  cyanids 
suitable  for  fumigation  work  should  be  practically  free  of  sodium 
chlorid. 

The  serial  numbers  6523-6529  in  the  table  on  page  92,  Part  III  of 
this  bulletin,  are  samples  of  cyanids  which  have  been  used  to  some 


i  Bui.  15,  Ga.  State  Bd.  of  Ent.,  1905. 
67330°— Bull.  90—12  7 


88  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

extent  in  fumigation  work  in  California.  An  examination  of  the 
data  in  this  table  shows  that  only  Nos.  6523,  6525,  6527,  and  6528 
are  suitable  for  use  in  fumigation.  Each  of  these  is  a  high-grade 
article  and  each  contains  less  than  1  per  cent  of  sodium  chlorid. 
The  other  three  samples  contain  greatly  in  excess  of  1  per  cent.  It 
should  be  noticed  that  these  last  three  samples  were  sold  as  98  to 
100  per  cent  sodium  cyanids. 

Whenever  sodium  cyanids  have  been  used  in  the  past,  the  same 
dosages  have  been  scheduled  as  for  the  regular  potassium  cyanid. 
Inasmuch  as  decomposition  was  then  unknown,  and  no  allowance 
was  made  for  it,  the  strength  of  gas  given  off  was  much  less  than 
was  believed  to  be  the  case.  The  result  has  been  poor  work.  This 
explains  the  past  unsatisfactory  work  with  the  11  American,"  or  98 
to  100  per  cent,  sodium  cyanid. 

THE  KIND  OF  CYANID  TO  PURCHASE. 

The  results  of  these  experiments  direct  attention  to  a  second 
consideration  in  the  purchasing  of  a  cyanid.  That  a  cyanid  be  of  a 
certain  grade  of  purity  is  no  longer  the  only  consideration.  It  is  of 
equal  importance  that  it  be  practically  free  of  sodium  chlorid.  The 
writer  would  condemn  as  unfit  for  use  in  fumigation  any  cyanid 
containing  in  excess  of  1  per  cent  of  sodium  chlorid. 

Returning  to  a  high-grade  sodium  cyanid,  it  has  been  found  on 
analysis  of  several  samples  that  these  contain  only  a  fraction  of  1 
per  cent  of  sodium  chlorid.  It  generally  can  be  held  as  a  safe  con- 
sideration that  a  cyanid  approaching  chemical  purity  will  contain 
not  more  than  a  trace  of  sodium  chlorid,  and  that  such  a  cyanid  can 
be  safely  used,  even  though  the  degree  of  cyanid  purity  is  alone  known. 
The  writer  would  consider  as  generally  satisfactory  for  fumigation 
work  a  sodium  cyanid  124  per  cent  pure  or  above.  A  chemical  of 
lower  purity  should  never  be  used.  Preferably  the  grade  demanded 
should  be  a  little  higher  than  that  given,  or  126  to  130  per  cent  pure. 
Absolute  chemical  purity  in  a  commercial  cyanid  can  not  be  expected, 
but  cyanids  of  the  degree  of  purity  recommended  herein  not  only 
are  within  reasonable  limits  of  expectation  but  should  be  demanded. 
When  chemicals  of  this  degree  of  purity  are  used,  analysis  for  sodium 
chlorid  is  unnecessary.  It  is  the  lower  grade  of  sodium  cyanid — the 
grade  "  100  per  cent  pure"  or  less,  which  contains  the  large  amounts 
of  sodium  chlorid. 

DOSAGES  WITH  SODIUM  CYANID.1 

All  recent  dosage  recommendations  in  fumigation  have  been  based 
on  a  high-grade  potassium  cyanid.  Such  a  situation  renders  it  neces- 
sary to  revise  our  present  schedules  should  we  desire  to  usesodium 


1  Whenever  sodium  cyanid  is  mentioned  in  this  bulletin,  a  high-grade  article,  one  124  to  130  per  cent 
pure,  is  meant  unless  otherwise  specified. 


BjI.  90,  Part  II,  Bureau  of  Entomology.  U.  S.  Dept.  of  Agriculture. 


O/ST^A/CEI   A/?OUA/D ;    /A/  rETETT. 


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OOSAGJET.  SCHEDULE  s4 ,   ro/?   rt/GH   Grt/IDE  SOO/UM    CK4A//D.    (/A/ OUA/CES).  COWG/A/AL). 


90,  Part  ll.  Bureau  of  Entomology.  U.  S.  Dept.  of  Agrici 


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SCHEDULE  ^ 


-A,  EO#  */Grt  GMDE  SO0/UM  CMA//0.    (/A/ OMCESj.  C°^/^fr± 


SODIUM  CYANTD  FOR  FUMIGATION  PURPOSES. 


89 


cyanid.  This  revision  is  accomplished  by  using  a  high-grade  sodium 
cyanid  with  dosages  one-fourth  less  than  the  amounts  scheduled 
for  the  potassium  salt.  Thus  if  a  dosage  recommended  should  read 
that  it  requires  12  ounces  of  potassium  cyanid  to  secure  certain 
result b  against  a  particular  insect  the  same  results  could  be  accom- 
plished with  9  ounces  of  a  high-grade  sodium  product.  In  general 
fumigation  against  the  purple  scale  (Lepidosaphes  hecVii  Newm.) 
the  writer  has  recommended  a  full  schedule  No.  1  of  potassium 
cyanid.1  Should  one  desire  to  produce  the  same  results  against  this 
insect  with  sodium  cyanid,  he  would  use  a  schedule  one-fourth  less,  or 
d<)>age  equal  to  three-fourths  of  the  dosage  amounts  given  in  schedule 
No.  1  of  potassium.  Such  a  three-fourths  schedule  with  high-grade 
sodium  cyanid  will  produce  practically  the  same  amount  of  gas  as  the 
full  schedule  No.  1  of  potassium. 

Plate  IX  gives  such  a  schedule,  which  is  called  schedule  A  for 
sodium  cyanid.1  By  using  this  schedule  with  a  124  to  130  per  cent 
cyanid  of  sodium  practically  the  same  results  will  be  secured  as  with 
full  schedule  No.  1,  as  given  in  Part  I  of  this  bulletin,  with  potassium 
cvan  id. 

These  dosages  are  in  ounces  and  are  based  on  a  knowledge  of  two 
measurements  of  the  tree — the  distance  around  the  bottom  and  the 
longest  distance  over  the  top.  Having  secured  these  two  distances 
for  anv  t  ree,  t  lie  dosage  for  that  tree  may  be  found  in  that  part  of  the 
table  where  the  lines  for  these  measurements  intersect. 

DOSAGES  RECOMMENDED  FOR  SCALE  PESTS. 

Pur  jilt  scale. — In  general  fumigation  for  the  purple  scale  (Lepido- 
BCkphes beckii  Newm.)  use  full  schedule  A  of  sodium  cyanid  (Plate  IX). 
With  its  use  all  insects  and  more  than  99  per  cent  of  the  eggs  are 
destroyed  on  the  leaves  and  branches.    Exposure  should  last  1  hour. 

Red  scale. — For  the  red  scale  (ChrysompJialus  aurantii  Mask.) 
use  a  three-fourths  schedule  (three-fourths  of  schedule  A  of  sodium 
cyanid  [Plate  X]).    An  exposure  of  45  minutes  is  sufficient. 

Villou:  scale — For  general  work  against  the  yellow  scale  (Chry- 
§omphdUn  citrinus  Coq.)  three-fourths  of  schedule  A  of  sodium 
cyanid  (Plate  X)  will  prove  satisfactory.  The  section  in  which  the 
yellow  scale  is  most  serious  is  about  Redlands,  San  Bernardino  County, 
where  a  great  acreage  is  planted  on  terraced  land.  Owing  to  the 
irregular  topography  of  terraced  orchards  the  tents  seldom  lie  as 
closely  to  the  ground  as  on  level  land,  and  as  a  consequence  there 
usually  is  a  greater  amount  of  leakage.  For  terrace  fumigation 
full  schedule  A  (Plate  IX)  should  be  used.  Exposure  should  be  45 
minutes. 


1  For  detailed  use  of  such  schedule,  as  well  as  methods  of  measuring  trees,  see  pp.  34-37,  Part  I,  of  this 
Bulletin. 


90  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


Black  scale. — When  the  black  scale  (Saissetia  olese  Bern.)  is 
partly  grown,  or  before  the  insects  become  tough  and  leathery, 
three-fourths  of  schedule  A  (Plate  X)  will  be  most  economical  for 
use.  However,  when  many  of  the  insects  are  full  grown  or  nearly  so, 
full  schedule  A  (Plate  IX)  is  recommended.  The  proper  time  to 
fumigate  for  the  black  scale  is  while  the  insects  are  in  the  earlier 
stages  of  development,  i.  e.,  before  they  become  tough  and  leathery. 
Exposure  should  be  1  hour  if  any  eggs  are  present.  Where  only 
young  scales  are  present,  45  minutes  is  sufficient. 

COMPARISON  OF  SODIUM  CYANID  AND  POTASSIUM  CYANID  FOR 
GENERAL  FUMIGATION. 

A  perusal  of  the  discussion  thus  far  ventured  in  this  bulletin  has 
shown  that  the  results  with  the  use  of  a  high-grade  sodium  cyanid 
were  equally  as  satisfactory  as  with  a  high-grade  potassium  cyanid. 
A  pound  of  the  sodium  cyanid  contains  at  least  one-fourth  more 
available  gas  than  a  pound  of  the  potassium  cyanid.  Hence  if  we 
pay  one-fourth  more  per  pound  for  the  high-grade  sodium  cyanid  than 
for  the  high-grade  potassium  cyanid,  the  ultimate  cost  of  fumigating 
an  orchard  will  be  practically  the  same  in  either  case ;  or  the  cost  of 
potassium  cyanid  at  24  cents  per  pound  is  equivalent  to  sodium 
cyanid  at  30  cents.  If  sodium  cyanid  (124  to  130  per  cent)  does  not 
cost  one-fourth  more  per  pound  than  potassium  cyanid  (98  to  99  per 
cent),  there  is  an  economy  in  the  use  of  the  former.  At  the  present 
prices  in  California  potassium  cyanid  costs  25  to  25^  cents  per  pound, 
whereas  the  sodium  cyanid  costs  practically  28  cents  per  pound. 
This  means  an  economy  of  2  to  3  cents  per  pound  in  favor  of  the 
sodium  cyanid. 

The  writer  recommends  a  124  to  130  per  cent  sodium  cyanid  as 
strongly  as  a  98  to  99  per  cent  potassium  cyanid  for  fumigation 
purposes.  The  sole  question  to  decide  between  the  use  of  these 
two  cyanids  in  any  particular  case  is  the  cost.  When  it  is  considered 
that  the  present  manufacture  of  sodium  cyanid  is  more  universal 
and  greatly  in  excess  of  the  potassium  cyanid;  that  the  sodium  com- 
pounds required  in  the  manufacture  of  sodium  cyanid  are  widely 
distributed  through  the  world,  while  commercial  deposits  of  the 
potassium  compounds  required  in  the  manufacture  of  potassium 
cyanid  are  largely  confined  to  the  German  Empire;  and  that  the 
present  unit  price  of  sodium  cyanid  averages  slightly  less  than  that 
of  the  oilier,  il  may  be  reasonably  expected  that  at  no  very  distant 
time  the  sodium  cyanid  may  be  found  supplanting  the  potassium 
cyanid  for  fumigating  purposes. 


U.  S.  D.  A.,  B.  E.  Bui.  90,  Part  III. 


Issued  May  10,  1911 


HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


CHEMISTRY  OF  FUMIGATION  WITH  HYDROCYANIC-ACID 

GAS.1 

By  0.  C.  McDon  \  BIX, 
Chief,  Insecticide  and  Fungicide  Laboratory,  }fisceUaneous  Dit  ision .  Bureau  of  Chemistry. 

INTRODUCTION. 

The  Miscellaneous  Division  of  the  Bureau  of  Chemistry  was 
requested  by  Mr.  ('.  L.  Marlatt,  Assistant  Chief  of  the  Bureau  of 
Entomology,  to  analyze  certain  chemicals  which  were  being  ussd 
by  Mr.  R.  S.  Woglum,  in  charge  of  the  field  work  with  hydrocyanic- 
acid  gas  in  southern  California,  and  at  the  same  time  to  investigate 
certain  chemical  problems  in  connect  ion  with  this  work  which  Mr. 
Woglum  had  outlined. 

In  view  of  the  great  importance  of  the  subject  from  an  economic 
standpoint  and  the  small  amount  of  work  that  has  been  done  in  the 
studv  of  the  chemical  problems  involved,  the  question  has  been  thor- 
oughly investigated.  The  action  of  mineral  acids  on  cyanids  and 
hydrocyanic  acid  has  been  quite  thoroughly  studied  by  a  number  of 
chemists,  but  the  important  bearing  of  this  point  on  the  question  of 
fumigation  has  not  been  brought  to  the  attention  of  those  engaged 
in  this  work  with  the  force  which  it  demands. 

I.  ANALYSES   OF   CHEMICALS   USED   FOR  THE   PRODUCTION  OF 
HYDROCYANIC-ACID  GAS. 

SULPHURIC  ACID. 

Ordinary  commercial  sulphuric  acid  is  usually  sold  of  the  strength 
known  as  66°  Baume,  which  corresponds  to  a  pure  product  containing 
96  per  cent  of  sulphuric  acid  (II2S04).  The  specific  gravity  of 
commercial  acid,  owing  to  the  presence  of  impurities,  is  always 
higher  than  that  of  the  pure1  acid,  and  commercial  acid  of  this  grade 
will  not  average  over  93  or  94  per  cent  of  sulphuric  acid.    A  sample 

1  A  detailed  outline  of  experiments  covering  certain  problems  in  the  chemistry  of  hydroeyanic-acid  gas 
fumigation  was  submitted  by  the  writer  (o  the  Bureau  of  Chemistry  for  execution.  This  work  was  carried 
out  under  the  direction  of  Dr.  J.  K.  Haywood,  Chief  of  the  Miscellaneous  Division,  by  Mr.  C  C.  McDonnell, 
Chief  of  the  Insecticide  and  Fungicide  Laboratory.  A  report  by  Mr.  McDonnell  covering  the  results  of  this 
chemical  investigation  was  sent  to  the  Bureau  of  Entomology,  August,  1909,  and  is  here  given.  Certain 
important  considerations  included  in  this  report  have  already  been  made  public— R.  S.  Woglum. 

91 


92  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


of  commercial  acid  forwarded  by  Mr.  Woglum  had  the  following 
composition : 

No.  6530:  Specific  gravity,  1.827  (65.4°  Baume),  91.21  per  cent 

H2S04. 

It  contained  merely  a  trace  of  ferrous  sulphate,  and  no  chlorids 
or  nitrates  were  present.  This  is  a  fairly  representative  sample  of 
commercial  acid.  A  trace  of  nitric  acid  is  sometimes  present  in 
commercial  sulphuric,  but  the  amount  would  probably  never  be 
large  enough  to  be  of  any  consequence  when  used  for  fumigation 
purposes. 

CYANID  SAMPLES. 

Certain  cyanid  samples  received  from  Mr.  Woglum  have  been 
analyzed  for  cyanogen  and  chlorin.  Practically  all  commercial 
cyanids  contain  small  amounts  of  cyanates,  carbonates,  and  traces 
of  other  compounds,  but  none  of  them  has  been  determined  quan- 
titatively in  these  samples,  as  their  presence  in  small  amounts  does 
not  interfere  with  the  use  of  these  substances  for  fumigation. 


Table  of  analyses. 


Serial 
No. 

Material. 

Total 
HCN. 

Calculated 
to  NaCN. 

Calculated 
to  KCN. 

Chlorin, 
calculated 
to  NaCl. 

6523 
6524 
6525 
6526 
6527 
6528 
6529 

Potassium  cvanid  "98-99  per  cent  pure"  

Sodium  cyahid  "98-99  per  cent  pure"  

Sodium  cyanid  "130  per  cent  pure"  

Sodium  cvanid  1  

Potassium  cyanid  "98-99  per  cent  pure"  

Potassium  cyanid  "98-99  per  cent  pure"  

"Sodium  cyanid"  2  

Per  cent. 
40.  42 
41.78 
51.22 
41.45 
39. 96 
39.28 
41.02 

Per  cent. 

75.78 
92.92 
75. 18 

74. 41 

Per  cent. 
97.41 
100.68 
123.44 
99.88 
96.30 
94.  67 
98.85 

Per  cent. 
0.40 
14.20 
.57 
5.82 
.60 
.77 
6.15 

1  A  mixture  of  potassium  and  sodium  cyanids  (potassium  equivalent  to  21.03  per  cent  KCN). 

2  A  mixture  of  potassium  and  sodium  cyanids  (potassium  equivalent  to  57.92  per  cent  KCN). 


The  purity  of  sodium  cyanid  is  frequently  stated  in  terms  of  potas- 
sium cyanid.  100  parts  of  sodium  cyanid  being  equivalent  to  132.85 
parts  of  potassium  cyanid;  that  is,  100  parts  of  sodium  cyanid  will 
yield,  theoretically,  as  much  hydrocyanic-acid  gas  as  132.85  parts  of 
potassium  cvanid. 

AH  of  these  results  are  calculated  to  the  potassium  cyanid  equiva- 
lent for  comparison.  Samples  Nos.  6527  and  6528  had  been  exposed 
to  the  air  for  three  months  or  longer  before  they  were  received  and 
were  undergoing  slow  decomposition,  as  shown  by  the  odor  of  ammo- 
nia which  they  possessed.  Decomposition  would  have  been  much 
more  rapid  than  here  shown  if  the  samples  had  been  exposed  to  moist 
air.  Cyanids  should  always  be  kept  dry  and  out  of  contact  with  the 
air  to  prevent  their  decomposition.  From  the  table  of  analyses 
it  will  be  seen  that  samples  Nos.  6524,  6526,  and  6529  contain  con- 


C  1 1  KM  ISTHY   OF  FUMIGATION. 


93 


siderable  amounts  of  sodium  chlorid.  While  the  amount  ofc\-anid 
present  is  equivalent  to  that  found  in  a  high-grade  potassium  cyanid, 
nwing  to  the  presence  of  this  chlorid  they  will  yield  considerably  less 
gas,  as  subsequently  shown  in  this  paper,  and  are  therefore  not  as 
valuable  for  fumigating  purposes. 

II.  PROPORTION  OF  CYANID,  SULPHURIC  ACID,  AND  WATER  FOR 

BEST  YIELD  OF  GAS. 

POTASSIUM  OTA  NTD, 

When  dilute  sulphuric  acid  acts  on  potassium  cyanid,  resulting  in 
complete  decomposition,  the  theoretical  reaction  is  as  follows: 

L>IvC\  +  ir2so4  =  K2so4  +  2ii(\\. 

According  to  this  reaction  1  ounce  (avoirdupois)  of  potassium 
cyanid  (KCN  100  percent)  would  require  0.75  ounce  (avoirdupois) 
of  sulphuric  acid,  H2S04,  or  0.S1  ounce  (avoirdupois)  of  commercial 
sulphuric  acid  containing  03  per  cent  sulphuric  acid,  which  would  be 
equal  to  0.42  fluid  ounce. 

The  above  reaction  can  not  be  obtained  under  the  conditions  exist- 
ing in  fumigation  work,  and  to  get  the  best  yield  of  gas  it  is  necessary 
to  have  a  considerable  excess  of  sulphuric  acid.  Under  such  condi- 
tions it  would  be  the  acid  potassium  sulphate  that  would  be  formed 
and  the  reaction  would  proceed  thus: 

KCN  +  II2S04  -  KIIS()4  4-HCN, 

or  for  each  ounce  (avoirdupois)  of  potassium  cyanid  there  would  be 
required  0.84  fluid  ounce  of  93  per  cent  sulphuric  acid.  This  amount 
has  been  found  in  practice  to  give  the  best  results,  or,  in  round  numbers, 
for  convenience  in  practical  Geld  work,  1  part  cyanid  to  1  part  acid. 
According  to  laboratory  work  the  proportion  of  water  that  should  be 
used  with  these  amounts  of  cyanid  and  acid  in  order  to  obtain  the 
best  yield  of  gas  w  as  2  parts.1  In  field  practice,  however,  Mr.  Wog- 
lum  lias  pointed  out2  that  it  is  not  always  practicable  to  use  this 
amount  on  account  of  the  fact  that  the  residue  sometimes  solidifies 
in  the  generating  jar,  and  he  has  therefore  adopted  and  recommends 
:>  parts  of  w  ater,  or  a  1-1-3  formula. 

SODIUM  CYANID. 

The  action  of  sulphuric  acid  on  sodium  cyanid  is  identical  with  its 
action  on  the  potassium  salt,  and  is  as  follows: 

2  X  aCN  +  II2S04  =  Na2S04  +  2HCX. 

'  Hill.  79,  Bur.  Ent.,  U.  S.  Dept.  Agr.,  pp.  37-38, 1909. 
2  Ibid.,  p.  39. 


94  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


As  sodium  has  a  lower  atomic  weight  than  potassium,  a  greater 
yield  of  gas  is  obtained  from  the  same  weight  of  the  cyanid  of  the 
former  and  a  larger  amount  of  sulphuric  acid  is  required  for  its 
decomposition.  According  to  this  reaction,  1  ounce  (avoirdupois) 
of  sodium  cyanid  requires  1  ounce  (avoirdupois)  of  sulphuric  acid, 
H2S04,  or  1.07  ounces  of  commercial  sulphuric  acid  containing  93 
per  cent  of  sulphuric  acid,  which  is  equivalent  to  0.56  fluid  ounce. 
As  before  noted,  to  get  the  best  yield  of  gas  the  sulphuric  acid  must 
be  in  considerable  excess,  when  the  reaction  would  be: 

NaCN  +  H2S04  =  NaHS04  +  HCN, 

or  for  each  ounce  of  sodium  cyanid  there  would  be  required  2.14 
ounces  (avoirdupois)  of  93  per  cent  sulphuric  acid,  equivalent  to  1.12 
fluid  ounces.  To  determine  the  best  proportions  of  sodium  cyanid, 
sulphuric  acid,  and  water  to  use  in  actual  practice  in  order  to  obtain 
the  largest  yield  of  gas,  the  following  experiments  were  carried  out, 
using  pure  sodium  cyanid  containing  94  per  cent  actual  sodium 
cyanid,  the  remainder  being  mainly  moisture,  and  commercial  sul- 
phuric acid  "66°  Baume,"  which  on  analysis  showed  93.52  per  cent 
sulphuric  acid.  Three-ounce  (avoirdupois)  portions  of  the  sodium 
cyanid  were  employed  in  each  experiment  and  varying  amounts  of 
sulphuric  acid  and  water  were  taken.  The  decomposition  was  car- 
ried out  in  tall  beakers  of  4-liter  capacity  in  order  to  prevent  any 
possibility  of  loss  from  spattering.  The  water  was  first  measured 
into  the  beaker,  then  the  acid  added,  and  the  weighed  package  of  cyanid 
was  immediately  dropped  in.  After  standing  40  minutes  the  residue 
was  washed  out  of  the  beaker  into  a  graduated  flask,  cooled,  made  up 
to  mark,  thoroughly  mixed,  and  aliquots  taken  for  the  determination 
of  the  amount  of  hydrocyanic  acid  remaining  in  solution.  As  has 
been  shown  by  experiments  herein  reported,  the  difference  between 
the  total  amount  of  cyanid  in  the  quantity  of  material  taken  and 
that  remaining  in  the  residue  does  not  represent  correctly  the  amount 
of  gas  given  off  and  available  For  fumigating  purposes,  but  when 
operating  on  a  pure  cyanid,  the  less  hydrocyanic  acid  remaining  in 
the  solution  the  greater  will  be  the  yield,  and  for  measuring  the  rela- 
tive efficiency  of  the  different  mixtures  of  acid  and  water  it  is  only 
neeessajy  to  del  ermine  the  amount  of  hydrocyanic  acid  in  the  residue. 

In  all  determinations  of  cyanid  Liebig's1  method  has  been  used. 
In  every  ease  where  ammonia  was  present  in  the  solution,  due  to 
previous  decomposition  of  the  cyanid  (in  which  case  the  end  rencl  ion 
would  not  appeal'  as  soon  as  it  should,  due  to  the  solvent  action  of 
ammonia  on  the  silver  cyanid),  a  few  drops  of  a  dilute  solution  of 
potassium  iodid  were  added  in  order  to  overcome  this  source  of  error, 
silver  iodid  being  insoluble  in  dilute  ammonia. 


1  Volumetric  Analysis,  Sutton,  9th  e<l.  rev.,  p.  200. 


CHEMISTRY  OF   FUMIGATION.  95 


Experiments  to  determine  the  best  proportion  of  chemicals  to  be  used. 


Experiment  No. 

A  mnii n 
-V  IJ1UUI 

Sodium 
cyanic! 
I  NaCN  . 

t  oi  cneinicu 

Sulphuric 

acid 
(H»S04). 

s  used. 

Water 

(H20). 

HCN  re- 
maining in 
residue 
(per  cent 
of  total). 

HCN  ex- 
pelled (by 
difference). 

Oz.,  avoir. 

Fluid  oz. 

Fluid  n:. 

Percent. 

1  

3 

3 

3 

12. 94 

87.06 

2  

3 

3 

4 

17.58 

82.42 

3  

3 

3 

5 

10. 19 

s«».sl 

4  

3 

3 

6 

7.48 

92.52 

5  

3 

3 

9 

9. 18 

90.82 

6  

3 

4 

4 

1.44 

98.56 

7  

3 

4 

6 

2.  49 

97.51 

8  

3 

4 

8 

4. 36 

95.64 

9  

3 

4 

10 

7. 53 

92.  47 

3 

4 

12 

8.14 

91.86 

11  

3 

5 

5 

1.40 

'.'V  60 

12  

3 

5 

7i 

4.04 

95.96 

13  

3 

5 

10 

5. 92 

94.08 

14  

3 

5 

15 

9. 91 

90.09 

IS  

3 

6 

6 

1.22 

98.78 

16  

3 

6 

8 

2. 74 

97.26 

17... 

3 

6 

10 

3.40 

96.54 

18  

3 

6 

12 

5.00 

95.00 

The  following  experiments  were  then  made  on  samples  of  sodium 
cyanid  of  varying  composition:  No.  6524  contained  75.78  per  cent  of 
sodium  cyanid  and  14.20  per  cent  of  sodium  chlorid;  No.  6525  con- 
tained 92.92  of  sodium  cvanid;  and  No.  6526  contained  75.18  per 
cent  calculated  as  sodium  cvanid  and  5.82  per  cent  of  sodium  chlorid. 


Experiments  n  ith  different  samples  of  sodium  cyanid. 


Experiment  No. 

Amount  of  c  hemicals  used. 

HCN  remaining  in  residue 
(per  cent  of  total). 

Sodium 
cyanid. 

Sulphuric 
acid. 

Water. 

No.  6524. 

No.  6525. 

No.  6526.1 

1»  

Oz.,aroir. 
3 

Fluid  o:. 

3 

Fluid  oz. 

3 

10.  74 

12.22 

8.53 

20  

3 

3 

7...S 

8. 16 

7.22 

21  

3 

3 

I 

7.27 

9.  32 

22  

3 

* 

.62 

3. 16 

.80 

23  

3 

\ 

1.65 
.39 

3.21 

24  

3 

1.73 

.53 

25  

3 

6 

.24 

2.28 

.58 

1  A  mixture  of  sodium  cyanid,  potassium  cyanid.  and  sodium  chlorid. 


The  acid  acts  very  energetically  on  the  sodium  cyanid,  and  by 
comparing  the  results  with  those  obtained  in  previous  work  with 
potassium  cyanid  it  is  shown  that  less  hydrocyanic  acid  remains  in 
the  residue  when  using  sodium  cyanid.  It  may  be  stated  here  that 
the  very  low  results  obtained  with  the  impure  cyanids  Nos.  6524  and 
6526  are  due  to  the  decomposing  action  of  hydrochloric  acid,  liberated 
from  the  sodium  chlorid,  on  the  hydrocyanic  acid.  When  equal 
quantities  of  cyanid,  acid,  and  water  were  used,  there  was  not  enough 
acid  present  to  cause  complete  decomposition  of  the  cyanid — decom- 
posed lumps  of  which  always  remained  in  the  residue — nor  was 


96  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

there  sufficient  water  to  readily  dissolve  the  sodium  sulphate  formed, 
as  a  result  of  which  the  residue  in  the  beaker  became  semisolid. 
With  the  proportions  3-4-4,  3-5-5,  and  3-6-6,  less  than  2  per  cent  of 
the  total  amount  of  hydrocyanic  acid  yielded  by  the  cyanid  remained 
in  the  residue  when  operating  on  a  pure  salt.  There  is  little  difference 
in  the  results  from  the  three  proportions,  and  4  fluid  ounces  of  acid  to 
3  ounces  (avoirdupois)  of  cyanid  appear  to  be  sufficient.  It  is  not 
necessary  to  have  as  much  water  present  to  keep  the  residue  in  the 
jar  from  u freezing"  as  when  potassium  cyanid  is  used,  because  the 
sodium  sulphate  formed  is  more  soluble  than  potassium  sulphate. 
There  was  no  trouble  from  this  cause  in  the  use  of  the  3-4-4  formula. 
These  experiments  were  carried  out  on  a  warm  day,  and  the  vessels 
were  placed  on  a  tin  roof.  If  they  were  placed  on  the  ground  under 
a  tent,  the  tendency  would  be  to  cool  more  rapidly,  and  under  such 
conditions  it  may  be  found  necessary  in  order  to  prevent  "freezing" 
to  increase  the  proportion  of  water,  though  it  is  desirable  to  keep 
this  as  low  as  possible,  as  an  excess  of  water  lowers  the  temperature 
of  the  acid  and  also  holds  in  solution  more  of  the  hydrocyanic-acid 
gas.  ^ 

Using*  the  3-4-6  formula,  which  contains  sufficient  acid  to  decom- 
pose all  of  the  cyanid  and  also  enough  water  to  readily  dissolve  all 
of  the  sodium  sulphate  formed  and  prevent  "freezing,"  only  about 
2\  per  cent  of  the  theoretical  yield  of  the  hydrocyanic  acid  remained 
in  the  residue.  The  following  formula,  therefore,  is  recommended  as 
a  good  one  for  practical  fumigation  work  in  the  field:  3  parts  sodium 
cyanid,  4  parts  acid,  and  6  parts  water — the  sodium  cyanid  being 
expressed  in  ounces  avoirdupois  and  the  acid  and  water  in  fluid 
ounces. 

III.  ACTION  OF  MINERAL  ACIDS  ON  CYANID S  AND  HYDROCYANIC 

ACID. 

The  reactions  which  take  place  when  sulphuric  acid  acts  on  potas- 
sium and  sodium  cyanids  have  been  given  and  the  amounts  of  hydro- 
cyanic acid  remaining  in  the  residue  after  mixing  these  substances 
under  certain  conditions  determined,  it  being  assumed  that  the 
remainder  was  given  off  and  available  for  fumigation  purposes. 
This  assumption,  however,  is  not  entirely  correct,  as  all  mineral 
acids  cause  more  or  less  decomposition  of  hydrocyanic  acid.  With 
sulphuric  acid  this  action,  under  the  conditions  in  which  fumigations 
arc  conducted,  amounts  to  very  little;  in  fact,  it  is  scarcely  worth 
considering,  but  in  the  case  of  an  impure  cyanid  containing  sodium 
chlorid  (a  frequent  impurity),  which  on  addition  to  sulphuric  acid 
liberates  hydrochloric  acid,  this  decomposing  action  becomes  a  very 
important  consideration  and  no  doubt  accounts  for  numerous  reported 
instances  of  failure  in  fumigation  work.    It  is  possible  to  conceive  of 


CHEMISTRY  OF  FUMIGATION. 


97 


Conditions  under  which  all  of  the  hydrocyanic  acid  may  be  decom- 
posed by  the  hydrochloric  acid  and  none  whatever  expelled  into  the 
medium  which  it  is  desired  to  fdl  with  the  gas. 

This  action  of  acids  on  hydrocyanic  acid  has  been  known  to  chem- 
ists for  years,  but  the  first  work  the  writer  has  seen  in  which  its  bearing 
upon  the  subject  of  fumigation  had  been  brought  to  the  attention  of 
the  public  was  by  Newell, 1  in  which  he  shows  the  decomposing  action 
of  hydrochloric  and  nitric  acids  on  hydrocyanic  acid  and  points  out 
the  necessity  of  using  pure  cyanids  for  fumigation  work. 

ACTION  OF  SULPHURIC  ACID  ON  HYDROCYANIC  ACID. 

When  potassium  cyanid  or  sodium  cyanid  is  treated  with  sulphuric 
acid, the firsl  action  which  takes  place  is  the  liberation  of  hydrocyanic- 
acid  gas,  as  shown  by  previous  equations.  This  is  then  acted  upon 
to  a  greater  or  less  extent,  depending  upon  the  conditions,  and  a 
portion  of  it  decomposed.  The  principal  products  of  decomposition 
aie  ammoniii  and  formic  acid,  according  to  the  following  reaction: 

IK  X    _MI_>0  =  IIC00II  +  X1I!. 

The  ammonia  formed  combines  with  the  excess  of  sulphuric  acid 
present  and  forms  ammonium  sulphate.  If  the  sulphuric  acid  is 
concentrated,  it  attacks  the  formic  acid,  extracting  water  therefrom 
and  liberating  carbon  monoxid,  CO,  thus: 

[h1coJ5h]  =  h2o  +  (o. 

Wade  and  Panting  :'  have  shown  that,  on  treating  potassium  cyanid 
with  sulphuric  acid,  by  suitably  varying  the  concentration  of  the 
acid  a  practically  quantitative  yield  of  either  hydrocyanic  acid  or 
carbon  monoxid  can  be  obtained.  With  dilute  sulphuric  acid  and  up  to 
a  si  length  )f  1  part  acid  to  1  part  water,  which  is  as  strong  as  it  is  ever 
used  in  fumigation  work,  nearly  pure  hydrocyanic  acid  is  formed. 
With  a  stronger  acid,  however,  ''a  certain  amount  of  carbon  monoxid 
is  formed,  and  as  the  concentration  of  the  acid  is  increased  the  volume 
of  gas  increases,  while  the  amount  of  hydrogen  cyanid  diminishes; 
and  finally,  when  ordinary  concentrated  sulphuric  acid  is  allowed  to 
act  on  the  cyanid,  nearly  pure  carbon  monoxid  is  evolved  in  almost 
theoretical  quantity."  Concentrated  sulphuric  acid  at  a  high  tem- 
perature is  reduced  by  hydrocyanic  acid,  sulphur  dioxid,  carbon 
dioxid,  and  ammonia  being  formed. 

I  [CN  +  H2S04  =  XH3  +  C02  +  S02. 
As  sulphuric  acid  stronger  than  1  part  acid  to  1  part  water  is  never 
used  in  fumigation  work,  we  need  not  concern  ourselves  with  the 


1  Georgia  State  Board  of  Entomology,  But.  15. 

2  Journ.  Chem.  Soc,  vol.  73,  pt.  I,  p.  255,  1898. 


98  HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


effects  produced  by  an  acid  of  greater  strength  than  this.  With  a 
more  dilute  acid  than  1  to  1  the  decomposing  action  on  hydrocyanic 
acid  amounts  to  very  little,  as  shown  by  experiments  which  are 
reported  herein. 

ACTION  OF  HYDROCHLORIC  ACID  ON  HYDROCYANIC  ACID. 

According  to  the  researches  of  Gautier  1  hydrocyanic  acid  com- 
bines directly  and  sometimes  violently  with  hydrochloric  acid,  result- 
ing in  bodies  comparable  with  the  ammonium  salts  or  the  amids. 
Gautier  found  that  on  passing  dry  hydrochloric-acid  gas  into  dry 
hydrocyanic-acid  gas,  then  heating  to  from  30°  to  40°,  sealed  in  glass 
vessels,  and  allowing  to  cool,  a  colorless  crystalline  substance  sepa- 
rated. From  the  results  of  the  analysis  of  this  compound  he  con- 
cluded that  the  formula  must  be  HCN,HC1.  He  found  that  this  body 
was  extremely  hygroscopic  and  underwent  partial  dissociation,  but 
decomposed  mainly  into  ammonium  chlorid  and  formic  acid  in  the 
following  manner: 

HCN,HC1  +  2H20  =  NH3C1  +  HCOOH 

Several  years  later  Claisen  and  Matthews  2  made  a  study  of  the 
action  of  hydrochloric  acid  on  hydrocyanic  acid,  and  by  employing 
a  different  method  also  obtained  a  white  crystalline  compound,  which 
conducted  itself  similarly  to  the  compound  obtained  by  Gautier,  but 
on  analysis  the  results  did  not  agree  with  the  formula  HCN,HC1  as 
found  by  him. 

Claisen  and  Matthews,  from  their  analytical  results,  ascribed  to 
the  compound  the  formula  2HCN,3HC1,  and  the  following  proper- 
ties: "A  colorless  crystalline  mass,  inodorous  in  dry,  and  fuming 
strongly  in  moist,  air.  It  appears  to  be  only  slightly  poisonous  and 
on  solution  in  water  shows  only  traces  of  hydrocyanic  acid,  probably 
present  as  an  impurity  in  the  original  compound.  It  is  insoluble  in 
ethylic,  formic,  or  acetic  ethers;  soluble,  but  with  decomposition,  in 
water  and  alcohol.  On  exposure  to  air  it  is  rapidly  converted  into 
ammonium  chlorid  and  formic  acid."  In  fact,  it  resembled  physi- 
cally and  gave  exactly  the  same  reactions  as  the  compound  obtained 
by  Gautier. 

More  recently  Nef  3  has  made  an  exhaustive  study  of  the  cyanogen 
compounds,  including  the  action  of  hydrochloric  acid  on  hydrocyanic 
acid.  He  arrived  at  the  conclusion  that  these  addition  products  are 
derivatives  of  imidoformylcyanid, 

rComptes  rendus,  vol.  66,  p.  410,  1807. 
i]  Annalen  der  Chemie  unci  I'harmacie,  vol.  145,  p.  118,  1808. 

I  Annates  <le  ohimie  et  de  physique,  ser.  4,  vol.  17,  p.  128,  1809. 
JJour.  Cham.  Soc.  vol.  41,  p.  204,  1882. 

[Beriohte  der  deutschen  ehemischen  Gesellsohaftj  vol.  10,  p.  308, 1883. 
3  Lieblg'fl  Annalen  der  Chemie  und  J'haruiaeie,  vol.  287,  pp.  205-359,  1895. 


CHEMISTRY  OF  FUMIGATION.  99 

(HCX)2  or  H — N=C — H. 
The  first  product  formed  being  imidoformylchlorid 

ir — x=c — n, 

I 

ci 

which  compound  immediately  combines  with  a  second  molecule  of 
HCX  thus 

UN  NH 

H — N=C— H  +  HCX  =      ^C— C^ 

forming  an  imid-chlorid.  This  in  turn  take-  up  one  or  two  molec- 
ular proportions  of  IIC1  with  the  formation  of  compounds  of  the 
following  constitut  ion : 


These  compounds  decompose  in  the  presence  of  moisture,  as  before 
described,  mainly  into  formic  acid  and  ammonium  chlorid. 

EFFECT  OF  THE  PRESENCE  OF  SODIUM  CHLORID  IN  CYANIDS  ON 
THE  YIELD  OF  HYDROCYANIC-ACID  GAS  IN  FUMIGATIONS.  1 

In  order  to  determine  the  actual  effect  of  different  proportions  of 
sodium  chlorid  (which  on  being  treated  with  strong  sulphuric  acid 
liberates  hydrochloric  acid)  in  cyanids,  when  used  for  fumigation 
work,  on  the  amount  of  hydrocyanic-acid  gas  liberated,  the  following 
experiments  were  conducted.  Samples  were  prepared  from  pure 
sodium  cyanid  and  sodium  chlorid  containing  various  proportions  of 
sodium  chlorid  from  9  to  66.66  per  cent. 

Several  experiments  were  also  conducted,  using  pure  sodium  cyanid, 
containing  94  per  cent  of  actual  sodium  cyanid  (the  remainder  mainly 
moisture),  for  the  purpose  of  determining  the  loss  when  using  pure 
chemicals.  These  experiments  were  carried  out  as  nearly  as  possible 
under  the  conditions  actually  obtaining  in  practical  fumigation  work, 


1  See  also  work  on  this  subject  by  Newell,  Georgia  State  Board  of  Entomology,  Bulletin  No.  15. 


100        HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


except,  of  course,  on  a  small  scale,  and  provision  was  made  for  the 
collection  of  the  liberated  hydrocyanic  acid  that  it  might  be  quanti- 
tatively determined.  Figure  13  shows  a  cut  of  the  apparatus  employed 
for  this  purpose.  On  account  of  the  inability  to  control  the  large 
volume  of  gas  which  is  so  rapidly  generated  when  the  solid  cyanid  is 
all  added  to  the  acid  at  one  time  (as  is  done  in  actual  practice)  it  was 
necessary  to  add  it  slowly,  and  in  order  to  do  this  the  charge  was  dis- ' 
solved  in  water. 

DESCRIPTION  OF  APPARATUS. 


"A"  is  a  flask  of  about  100  cc  capacity  into  which  the  chemicals 
are  placed  for  the  generation  of  the  hydrocyanic-acid  gas;  "B," 


Fig.  13.— Apparatus  used  in  the  decomposilion  of  cyanidsand  collection  of  the  liberated  hydrocyanic- 
acid  gas.  (Original.) 

iiC/>  and  "~D"  are  DrechseFs  gas  wash  bottles,  and  into  each  is  put 
about  100  cc  of  a  dilute  solution  of  potassium  hydrate  to  absorb  the 
gas.  Bottle  "F  "  contains  a  potassium  hydrate  solution  to  free  the 
air  that  is  drawn  through  from  carbon  dioxid,  which  decomposes 
cyanids  in  solution  when  passed  through  them.  "E"  is  a  condenser 
containing  cold  water  to  cool  the  gases  before  reaching  "B"  and 
thus  prevent  heating  the  solution  in  "B,"  which  would  cause  decom- 
position of  the  potassium  cyanid  formed. 

DETAILS  OF  MANIPULATION. 

To  "A"  add  10  cc  of  water,  then  10  cc  of  concentrated  sulphuric 
acid,  after  which  add  immediately  to  the  separator}'  funnel  5  grams 


CHEMISTRY  OF  FUMIGATION. 


101 


of  the  cyanid,  previously  dissolved  in  10  cc  of  water,  and  partially 
open  the  stopcock,  allowing  the  solution  of  the  cyanid  to  flow  slowly 
into  the  acid.  As  soon  as  the  stream  of  hydrocyanic-acid  gas  reaches 
"B,"  containing  the  potassium  hydrate,  it  is  absorbed  very  readily; 
in  fact,  so  readily  that  unless  the  flow  is  quite  rapid  the  potassium- 
hydrate  solution  rises  in  the  tube  and  in  several  instances  was  drawn 
back  into  flask  "A."  In  order  to  prevent  this  it  is  necessary  to  watch 
the  apparatus  constantly  and  not  permit  the  flow  of  gas  to  subside; 
or,  if  this  is  impossible,  to  close  the  pinchcock  on  the  tube  connecting 
the  two  flasks  for  an  instant  and  apply  suction.  As  soon  as  the  solu- 
tion of  the  cyanid  has  all  been  added  the  separators'  funnel  is  washed 
down  with  1  cc  of  water,  heat  is  applied  to  flask  "A,"  and  the  tem- 
perature of  the  acid  brought  to  110°  C,  which  is  about  the  average 
temperature  obtained  by  adding  I  part  acid  to  2  parts  water.  The 
heat  is  then  removed,  flask  "F"  attached,  suction  applied,  and  air 
drawn  through  th<>  apparatus  for  about  40  minutes.  According  to 
the  authorities,  air  alone  drawn  through  a  solution  of  a  cyanid 
causes  it  to  decompose,  but  this  action  is  very  slow  and  for  this  work 
can  be  neglected.  The  residue  in  "A"  is  washed  out  into  a  beaker, 
cooled,  made  alkaline  with  potassium  hydrate,  and  the  amount  of 
cyanid  present  determined  by  titration  with  tenth-normal  silver 
nit  l  ate.  The  solution  in  "B"  is  washed  out  into  a  500  cc  flask,  made 
to  the  mark,  and  aliquot  portions  used  for  the  determination  of  the 
cyanid  present.    Solutions  in  and  "D"  were  also  tested  for 

cyanid,  but  in  only  two  or  three  instances  was  any  present,  as  in  most 
cases  it  had  all  been  absorbed  in  "B.  " 

RESULTS  OF  EXPERIMENTS. 


The  results  of  the  experiments  conducted  as  outlined,  when  employ- 
ing pure  chemicals,  are  given  in  the  following  table: 

Results  of  experimental  icork. 


Experiment  No.— 

Equivalent 
of  HCN  in 
the  sam- 
ple. 

Per  cont  of 
total  HCN 
evolved. 

Per  cent  of 
total  HCN 
remaining 
in  "A." 

Per  cent  of 
total  1ICNT 

decom- 
posed (by 
ditlorence). 

Per  cent. 
51.82 
51.82 
51.82 
51.82 
51.82 

94. 81 
94.40 
93.77 
96.90 
96.28 

3.59 
3.29 
3.84 
2.20 
1.40 

1.60 
2.31 
2. 39 
.90 
2.32 

Average  

51.82 

95.23 

2.86 

1.91 

These  results  indicate  that  there  is  some  variation  in  the  amount 
of  hydrocyanic  acid  decomposed,  but  this  would  be  expected  from 
the  fact  that  the  experiments  are  subject  to  slight  variations  in  the 


102        HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


manipulation.  The  average  amount  decomposed  is  less  than  2  per 
cent  and  the  average  remaining  in  the  residue  is  practically  3  per  cent, 
which  shows  that  when  operating  with  pure  sodium  cyanid  and  sul- 
phuric acid  we  may  expect  to  obtain  close  to  95  per  cent  of  the  theo- 
retical yield  of  hydrocyanic-acid  gas  expelled  and  available  for 
fumigation  purposes.  Sodium  cyanids  were  then  prepared  containing 
varying  quantities  of  sodium  chlorid  and  the  determinations  made  in) 
the  same  way.    The  results  are  tabulated  as  follows: 


Experiment  No. — 

Equiva- 
lent of 
HCN  in 
sample. 

Sodium 
chlorid 
in  sam- 
ple. 

HCN 
evolved. 

HCN  re- 
maining 
in  resi- 
due. 

HCN  de- 
com- 
posed 

(by  differ- 
ence.) 

Per  ce 
Evolved. 

nt  of  total 

In  res- 
idue. 

HCN. 

Decom- 
posed. 

6  

7  

8  

9  

10  

Per  cent. 
47. 16 
43.18 
34. 55 
25.91 
17.27 

Per  cent. 
9 

16| 
33§ 
50 
66| 

Per  cent. 
41.96 
27. 42 
10. 57 
5. 85 
1.27 

Per  cent. 
0. 66 
1.71 
.37 
.05 
.10 

Per  cent. 
4.54 
14. 05 
23. 61 
25. 91 
15. 90 

88.98 
63.49 
30.61 
22.58 
7. 35 

1.40 
3.97 
1.06 
.21 
.63 

9.62 
32.54 
68. 33 
77.21 
92.09 

AMMONIA  FORMED  FROM  THE  DECOMPOSITION  OF  THE  CYANID. 

The  amount  of  ammonia,  existing  as  ammonium  sulphate,  in  the 
residue  in  flask  "A"  and  that  passed  over  into  "B"  was  determined 
in  several  cases  and  the  nitrogen  calculated  therefrom.  These 
amounts  added  to  the  calculated  amount  of  nitrogen  in  the  hydro- 
cyanic acid  recovered  in  "A"  and  "B"  correspond  almost  exactly 
with  the  theoretical  amount  of  nitrogen  in  the  quantity  of  cyanid 
employed.    The  distribution  of  nitrogen  was  as  follows: 

Distribution  of  nitrogen  in  the  residue. 


Determination. 


Experi- 
ment 7. 


Experi- 
ment 8. 


In  "A": 

Nitrogen  as  cyanid . . 

Nitrogen  as  ammonia 
In  "  B  ": 

Nitrogen  as  cyanid . . 

Nitrogen  as  ammonia 


Per  cent. 
0. 79 
49.84 

46.83 
2. 54 


Per  cent. 
1.05 
66. 82 

30.40 
1.73 


This  shows  that  one  of  the  principal  decomposition  products  is 
ammonia,  the  greater  part  of  which  is  held  in  solution  in  the  generating 
flask  by  the  excess  of  sulphuric  acid  as  ammonium  sulphate. 

Experiments  carried  out  on  the  samples  of  commercial  cyanids, 
the  analyses  of  which  are  given  on  page  92,  gave  the  following  results: 


CHEMISTRY  OF  FUMIGATION.  103 


Experiments  on  commercial  cyanids. 


Ex- 
peri- 
ment 

No. 

Serial 
No. 

Material. 

Rquiv- 
alent  of 
HCN  in 
sample. 

Sodium 
chlorid 
in  sam- 
ple. 

ncN 

evolved. 

HCN 
remain- 
ing in 
residue. 

HCN 
decom- 
posed 
(bv  dif- 
fer- 
ence). 

Per  cenl 
Evolved. 

of  total  HCN— 

In  resi-  Decom- 
due.  |  posed. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

Per  ct. 

11 

6523 

Potassium  cyanid  . . . 

40. 42 

0.40 

37.95 

1.22 

1.25 

93.88 

3.03 

3.09 

12 

BSM 

Sodium  c\anid  

41.7s 

14.20 

26.22 

1.32 

14.24 

62. 76 

3.17 

34.07 

13 

6525 

 do  

51.22 

.57 

48.31 

2.31 

.30 

94. 32 

5.09 

.59 

14 

»wi26 

 do.'  

41.45 

5.82 

36  93 

2.04 

2.48 

89.10 

4.92 

5.98 

15 

u>.'7 

Potassium  cvanid  

39. 96 

.60 

37.51 

1.71 

.74 

93.87 

4.27 

1.86 

16 

6528 

 do  

39.28 

.77 

36.  71 

1.40 

1.17 

93.46 

3.57 

2. 97 

17 

6529 

••  Sodium  cyanid  . 

41.02 

6. 15 

35.17 

1.84 

4.01 

85.75 

4.49 

9.76 

I  A  mixture  of  potassium  and  sodium  cyanids  and  sodium  chlorid. 


This  work  showa  the  great  variation  in  the  yield  of  hydrocyanic 
acid  obtained  when  using  samples  as  they  appear  upon  the  market, 
nearly  twice  as  much  being  obtained  from  sample  No.  6525  as  from 
No.  652  1 .  In  view  of  such  varying  results  as  these  it  is  not  surprising 
that  fumigation  has  so  often  proved  a  failure. 

EFFECT  OF  THE  PRESENCE  OF  SODIUM  NITRATE  IN  CYANIDS  ON 
THE  YIELD  OF  HYDROCYANIC-ACID  GAS. 

Experiments  carried  out  in  the  same  way,  using  the  same  pure 
sodium  cvanid.  to  which  varying  proportions  of  sodium  nitrate  had 
been  added,  gave  results  similar  to  those  in  which  sodium  chlorid  was 
present.  This  facl  is  of  no  practical  utility  in  so  far  as  fumigation 
work  is  concerned,  as  cyanids  do  not  contain  nitrates  as  an  impurity. 
Commercial  sulphuric  acid  may  contain  traces  of  nitric  acid,  but 
the  amount  is  so  minute  that  it  would  have  no  appreciable  effect  on 
the  results.  Aside  from  this,  the  action  of  such  an  energetic  oxidizing 
agent  as  nitric  acid,  in  the  presence  of  strong  sulphuric  acid,  upon 
cyanids  would  be  attended  with  some  danger.  The  results  of  these 
experiments  are  given  in  the  following  table: 


Effect  of  nitrates  on  the  yield  of  hydrocyanic-acid  gas. 


Experiment  No. 

Equiva- 
lent of 
HCN  in 
sample. 

Sodium 
nitrate 
in  sam- 
ple. 

HCN 
evolved. 

HCN  re- 
maining 
in  resi- 
due. 

HCN  de- 
com- 
posed (by 
differ- 
ence). 

Per  cent  of  total  HCN— 

Evolved. 

In  resi- 
due. 

Decom- 
posed. 

Percent. 

Per  cent. 

Per  cent. 

Per  cent. 

Per  cent. 

18  

47.16 

9 

39. 92 

1.26 

5.98 

84.66 

2.67 

12. 67 

19  

43.18 

161 

37.73 

1.32 

4. 13 

87. 39 

3.05 

9.  56 

20  

25.91 

50 

21.22 

1.14 

3.65 

81.51 

4.40 

14.09 

21  

17.27 

66§ 

13. 12 

.79 

3.36 

75.95 

4.56 

19.49 

The  presence  of  nitrates  exerts  a  very  decided  decomposing  action 
on  the  hydrocyanic  acid,  but  this  action  is  much  less  than  that  pro- 
duced  by  chlorids.    The  nitrogen  in  the  decomposed  cyanid  is  in 
this  case  also  converted  into  ammonia. 
67330°— Bull.  90—12  8 


104        HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 

SUMMARY. 

In  order  that  the  results  may  be  more  readily  compared  they  are  all 
presented  in  one  table,  as  follows: 

Comparison  of  results  obtained  on  evolution  of  hydrocyanic-acid  gas  under  different 

conditions. 


Ex- 
peri- 
ment 

No. 

Material. 

Sodi- 
um 
chlorid 
in  sam- 
ple. 

Equiv- 
alent of 
HCN  in 
sample. 

HCN 
evolved. 

HCN 
remain- 
ing in 
residue . 

HCN 
decom- 
posed 
(by  dif- 
fer- 
ence). 

Per  cent 
Evolved. 

of  total 

In  resi- 
due. 

HCN— 

Decom- 
posed. 

Ir^CT  Ct. 

ID 

Jr€T  Ct. 

J-^er  cent. 

Jrer  ct. 

r'er  ct. 

1—5  1 

_  ,. 

U.  UU 

51.  82 

49.  35 

1.  48 

0.  99 

95. 23 

2.  86 

1.  91 

6 

 do  

9. 00 

47. 16 

41. 96 

.66 

4.  54 

88.  98 

1.40 

9.62 

7 

 do  

16.66 

43. 18 

27.42 

1. 71 

14.05 

63.  49 

3.  97 

32.  54 

8 

 do  :  

33.  33 

34.  55 

10.  57 

.37 

23.  61 

30.  61 

1.06 

68.  33 

9 

 do  

50.00 

25.  91 

5.  85 

.05 

25.  91 

22.  58 

.21 

nl.  21 

10 

 do  

66.  66 

17.  27 

1.27 

.10 

15.90 

7.35 

.63 

92.  09 

11 

Potassium  cyanid  

.40 

40.42 

37.  95 

1.22 

1.  25 

93.88 

3. 03 

3.  09 

12 

Sodium  cyanid  

14.20 

41.  78 

26.  22 

1.32 

14.  24 

62.  76 

3. 17 

34.  07 

13 

 do  

.57 

51.22 

48.  31 

2.  31 

.30 

94.  32 

5.09 

.59 

14 

 do.  2  

5.82 

41.45 

36.93 

2. 04 

2.  48 

89. 10 

4.  92 

5.  98 

15 

.60 

39.  96 

37.  51 

1.71 

.74 

93.  87 

4.  27 

1.  86 

16 

 do  

.77 

39.  28 

36.71 

1.40 

1.17 

93.  46 

3.57 

2.  97 

17 

6. 15 

41.02 

35. 17 

1.84 

4.  01 

85.75 

4.49 

9.  76 

Sodi- 

um 

nitrate. 

18 

9.00 

47. 16 

39.  92 

1.26 

5.  98 

8*4.  66 

2.  67 

12.  67 

19 

16. 66 

43. 18 

37.  73 

1.  32 

4. 13 

87.  39 

3.05 

9.  56 

20 

50.00 

25.  91 

21.22 

1. 14 

3.  65 

81.51 

4.40 

14.  09 

21 

 do  

66.  66 

17.  27 

13. 12 

.79 

3.  36 

75.  95 

4.  56 

19.  49 

1  Average  of  five  determinations. 

2  A  mixture  of  potassium  and  sodium  cyanid  and  sodium  chlorid. 


These  experiments  show  conclusively  that  the  presence  of  chlorids 
and  nitrates  in  cyanids  which  liberate  hydrochloric  and  nitric  acid 
respectively,  together  with  hydrocyanic  acid,  on  treatment  with 
sulphuric  acid  cause  very  marked  decomposition  of  the  hydrocyanic 
acid.  The  effect  produced  by  hydrochloric  acid  is  much  more  marked 
than  that  produced  by  nitric  acid.  In  one  case  (experiment  10) 
over  92  per  cent  of  the  hydrocyanic  acid  was  decomposed  and  only 
a  little  over  7  per  cent  evolved.  This  is  a  larger  amount  of  sodium 
chlorid  than  would  ever  be  found  in  a  commercial  sample,  but  it  shows 
the  important  bearing  this  impurity  has  upon  the  results.  Practi- 
cally all  commercial  potassium  and  sodium  cyanids  contain  sodium 
chlorid  in  greater  or  less  amount.  Potassium  cyanid  is  frequently 
sold  as  "9S-99  per  cent  pure/'  which  in  reality  is  a  mixture  of 
potassium  cyanid,  sodium  cyanid,  and  sodium  chlorid  and  on 
analysis  may  show  even  100  per  cent  expressed  as  potassium  cyanid 
yet  there  may  be  several  per  cent  of  sodium  chlorid  present.  For 
fumigation  work  an  analysis  of  a,  cyanid  is  of  little  value  unless  the 
chlorin  content  is  also  determined.  In  order  that  satisfactory  results 
may  be  obtained  in  the  fumigation  of  trees  for  the  control  of  insects 


CHEMISTRY  OF  FUMIGATION. 


105 


the  amount  of  hydrocyanic- acid  gas  that  can  be  used  falls  within 
narrow  limits.  If  the  application  is  too  strong  serious  injury  will 
result  to  the  trees,  while  on  the  other  hand  if  too  weak  many  of  the 
insects  will  escape  the  poisonous  action  of  the  gas,  thus  necessitating 
a  second  fumigation  or  giving  inefficient  results.  It  is  therefore 
necessary  that  the  strength  and  quality  of  the  reagents  used  be 
known  and  that  the  conditions  under  which  the  work  is  done  be 
uniform.  When  these  points  are  fully  realized  by  entomologists  and 
orchardists  there  is  no  doubt  thai  hetter  and  more  satisfactory 
results  will  he  obtained. 


!  N  DEX. 


Page. 

Acids,  mineral,  action  on  cyanidsand  hydrocyanic  acid   90-99 

Ammonia  formed  by  decomposition  of  cyanid   102-103 

Ammonium  sulphate,  residue  from  decomposition  of  cyanid   102-103 

Bordeaux -distil  late  emulsion,  injury  through  fumigation  to  trees  sprayed  there- 
with  72-73 

Cactus.   (See  0  punt  hi  engdmanni, 

Carboy  with  handles  to  facilitate  pouring  acid  in  fumigation   25,26 

Cart.    iSee  Supply  eart.) 

Chart  for  dosage  (see  also  Dosage  schedule). 

how  to  use  it   34-37 

Chemicals  in  fumigation   40-51 

amount  in  very  small  dosages   48 

mixing  48-49 

most  economical  proportion   47-48 

proportion  when  sodium  cyanid  is  used   85 

Chemistry  of  fumigation  with  hydro  >  a  ni<  -a<  id  gas   91-105 

summary   104-105 

Chrysoyn phalus  aurantii.    (See  Scale,  red.) 

ntrinus.    (See  Scale,  yellow.) 

Citrus  fruits,  insect  enemies  in  California   7-10 

orchards  in  California,  extent  and  character   6-7 

trees,  fumigated,  appearance   73 

presence  of  old  scales  thereon   74 

fumigation.    (See  Fumigation  of  citrus  trees.) 

greater  susceptibility  of  some  varieties  than  others  to  injury  from 

fumigation   67 

gum  disease   66 

sprayed,  injury  to  those  fumigated   72-73 

unhealthy,  effects  of  fumigation  thereon   66-67 

Climatic  conditions  (see  also  Meteorological  elements). 

effect  on  scale  insects  of  citrus   76-77 

CoccineUa  caUfomica,  effect  of  fumigation  thereon   77 

CoccinellidaB.    (See  Ladybirds.) 

Cold,  effects  in  fumigation  of  citrus  trees   70-71 

Cyanid,  "American,"  not  potassium  cyanid  but  sodium  cyanid   84 

ammonia  formed  by  its  decomposition   102-103 

"German"   84 

of  potassium,  analyses  of  samples  used  for  production  of  hydrocyanic- 
acid  gas   92-93 

comparison  with  sodium  cyanid  for  general  fumigation. .  90 

cost   79 

dosages  of  sodium  cyanid  in  comparison  with  dosages  of 

former   88-89 

in  fumigation   40-41 

proportion  for  best  yield  of  hydrocyanic-acid  gas   93 

strength  of  sodium  cyanid  expressed  in  terms  thereof. .  85 

107 


108        HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


Page. 

Cyanid  of  sodium,  analyses  of  samples  used  for  production  of  hydrocyanic -acid 


gas   92-93 

comparison  with  potassium  cyanid  for  general  fumigation. .  90 
dosages  in  fumigation  compared  with  dosages  of  potassium 

cyanid   88-89 

first  published  suggestion  as  to  use  in  fumigation   83-84 

inauguration  of  experiments  by  Bureau  of  Entomology  and 

Bureau  of  Chemistry   84 

in  fumigation  of  citrus  trees,  field  tests   86-87 

kind  to  purchase  for  fumigation   88 

proportion  for  best  yield  of  hydrocyanic-acid  gas   93-96 

of  chemicals  when  used  for  fumigation   85 

used  in  fumigation   85 

strength  expressed  in  terms  of  potassium  cyanid   85 

value  for  fumigation  purposes   83-90 

Cyanids,  action  of  mineral  acids  thereon   96-99 

effect  of  presence  of  sodium  chlorid  on  yield  of  hydrocyanic-acid  gas 

in  fumigation   99-102 

nitrate  on  yield  of  hydrocyanic-acid  gas. .  103 

Derricks  for  fumigating  tents   20-21 

Distillate  emulsion  and  Bordeaux  mixture,  injury  through  fumigation  to  trees 

sprayed  therewith   72-73 

spray  against  citrus  scales,  efficiency  as  compared  with  fumigation   2 

Dosage  calculation  in  fumigation  of  citrus  trees   27-28 

factors  affecting  it  in  fumigation   52-53 

formula  for  measuring  fumigating  tents   28 

schedule  A,  for  high  grade  sodium  cyanid   88 

No.  1,  for  potassium  cyanid   34 

|,  for  potassium  cyanid   59 

J-A,  for  high  grade  sodium  cyanid   88 

under  improved  system  (with  potassium  cyanid)   34-37 

Dosages  for  scale  pests  in  general  (with  potassium  cyanid)   61 

"Firing"  effect  produced  in  orchards  by  purple  scale   8 

Fumigation  against  black  scale   59-60 

mealy-bug   63-64 

purple  scale   53-57 

difficulty  of  destroying  scale  on  fruit   56-57 

dosage  for  eradication  (with  potassium  cyanid).  55-56 

length  of  exposure   55 

two  successive  treatments   57 

red  scale   57-58 

dosage  with  potassium  cyanid   58 

yellow  scale   60-61 

chemistry  thereof   91-105 

summary   104-105 

generators,  device  for  covering  them   74-76 

of  citrus  trees   1-81 

apparatus   10-24 

appearance  of  fumigated  trees   73 

by  associations   5 

contract   4-5 

counties   5 

private  individuals  '.   5-6 


INDEX.  109 

Pago. 

Fumigation  of  citrus  trees,  cautions,  general   80 

chart  for  dosage  (see  also  Fumigation  of  citrus  trees, 
dosage  schedule). 

how  to  use  it   34-37 

chemicals   40-51 

amount  in  very  small  dosages   48 

mixing   48—49 

proportions,  most  economical   47—48 

proportion  when  sodium  cyanid  is  used..  85 
comparison  of  sodium  cyanid  and  potassium  cyanid 

for  general  fumigation   90 

cost   78-79 

cover  for  generators   74-76 

cyanid  of  potassium   40-41 

sodium,  kind  to  purchase   88 

proportion  used   85 

derricks  and  poles   20-21 

distribution  of  gas  within  a  tent   67 

dosage  calculation   27-28 

factors  affecting  it   52-53 

schedule  A.  for  high  grade  sodium  cyanid...  88 

No.  1,  for  potassium  cyanid   34 

I,  for  potassium  cyanid   59 

|— A,   for  high  grade  sodium 

cyanid   88 

under  improved  system  (with  potas- 
sium cyanid)   34-37 

dosages  for  scale  pests  in  general  (with  potassium 

cyanid)   61 

various  scale  pests, with  potassium  cyanid  51-61 

sodium  cyanid . .  88-90 

during  blossoming  period   64-65 

effect  of  presence  of  sodium  chlorid  on  amount  of  gas 

given  off   49-50 

on  ladybirds  (Coccinellidse)  and  Scutellista 

cyanea   77-78 

effects  of  cold   70-71 

heat   69-70 

light   71-72 

meteorological  elements   68-72 

moisture   68-69 

temperature   69-71 

winds   72 

on  different  varieties   67 

unhealthy  trees   66-67 

extent  to  which  it  is  now  practiced  in  California. . .  3-4 

factors  which  affect  dosage   52-53 

field  tests  with  sodium  cyanid   86-87 

for  physiological  effects   67-68 

fumigating  lemons   65-66 

generating  vessels   24 

cover  device   74-76 

historical   1-2 

improved  system   32-40 


110        HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


Page. 

Fumigation  of  citrus  trees,  improved  system,  advantages   37-38 

experience  with  it   38-40 

injury  to  sprayed  trees   72-73 

leakage  of  gas,  allowance  therefor  under  improved 

system   33-34 

McFadden  machine  for  hoisting  tents  over  trees   21-22 

supply  wagon   23-24 

nature  of  residue  resulting  from  generation  of  gas. .  50-51 

poles  and  derricks   20-21 

presence  of  old  scales  on  fumigated  trees   74 

procedure,  general   24-31 

improved  system   37 

old  method   30-31 

published  articles  and  addresses  by  R.  S.  Woglum..  81 

renewal  of  interest  in  California   2-3 

securing  measurements  around  and  over  tents   28-30 

securing  measurements  around  and  over  tents,  Mor- 
rill method   29-30 

sulphuric  acid   41-43 

amount  necessary   42-43 

proportion  used  with  sodium  cyanid . .  85 

supply  cart  and  supply  wagon-   22-24 

systems  employed  in  California   4-6 

tents   10-20 

*     bell   16 

gas-proofing   16-18 

marking   19-20 

mildew-proofing   18-19 

sheet   10-16 

amount  of  cloth  required  for  different- 
sized  tents   14-15 

construction   13-14 

experiments  with  new  tent  material. ...  12 

material  used   11 

new  tenting  material   11-12 

ring  attachments  and  reenforcements. . .  15-16 

sizes   11 

size  to  purchase   15 

what  cloth  to  use   12-13 

time  of  year   61-63 

water  as  a  factor   44-47 

correct  proportion  with  potassium  cyanid   47 

sodium  cyanid   85 

effect  of  different  proportions  on  amount  of 

available  gas   45-47 

effect  of  different  proportions  on  temperature 

of  gas   45 

while  fruit  is  small   65 

Fungus,  black,  following  infestation  of  citrus  fruits  by  mealy-bug   10 

sooty-mold,  following  infestation  of  oranges  by  black  scale   3,  9 

(las.   {See  Hydrocyanic-acid  gas.) 

(  i<  !h  rating  vessels,  for  fumigation  r   24 

cost   79 

rover  device   74-76 


INDEX.  Ill 

Page. 

"Gophers,"  injury  to  citrus  trees   66 

Greenhouses,  fumigation   2 

Gum  disease  of  citrus  trees   66 

Heat,  effects  in  fumigation  of  citrus  trees   69-70 

Jlemerobius  sp.,  enemy  of  mealy-bug  (Pseudococcus  citri)   63 

Hippodnmia  convergens,  effect  of  fumigation  thereon   77 

Hydrochloric  acid,  action  on  hydrocyanic  acid   98-99 

Hydrocyanic  acid,  action  of  hydrochloric  acid  thereon   98-99 

mineral  acids  thereon   96-99 

sulphuric  acid  thereon   97-98 

gas  (see  also  Fumigation). 

cautions  regarding  use   80 

chemicals  used  for  production,  analyses   91-93 

distribution  within  a  fumigating  tent   67 

fumigation,  chemistry  thereof   91-105 

summary   104-105 

leakage  through  tents,  allowance  therefor  under  im- 
proved system  of  fumigation   33-34 

nature  of  residue  from  generation   50-51 

proportion  of  cyanid  of  potassium,  sulphuric  acid,  and 

water  for  best  yield  of  gas   93 

yield  as  affected  by  presence  of  sodium  chlorid  in  cya- 

nids   99-102 

nitrate  in  cya- 

nids   103 

Icerya  purchasi,  fumigation  of  citrus  trees  therefor   1 

Insect  enemies  of  citrus  fruits  in  California   7-10 

"Jinglor"  attachment  for  sheet  tents,  purpose   16 

Labor  for  fumigation,  cost   79 

Ladybirds,  effect  of  fumigation  thereon   77 

Lemon  trees,  fumigation   65-66 

Lepidosaphes  beckii.    (See  Scale,  purple.) 

Light,  effects  in  fumigation  of  citrus  trees   71-72 

Linseed  oil,  use  for  gas-proofing  fumigating  tents   16,  17-18 

McDonnell,  C.  C,  paper,  "Chemistry  of  Fumigation  with  Hydrocyanic-Acid 

Gas"   91-105 

McFadden  machine  for  placing  fumigation  tents  on  trees   21-22 

supply  wagon  for  use  in  fumigation  of  citrus  trees   23-24 

Mealy-bug,  enemy  of  citrus  fruits  in  California   10 

fumigation   63-64 

natural  control   63 

Meteorological  elements  (see  also  Climatic  conditions). 

effects  on  fumigation  of  citrus  trees   68-72 

Mills,  fumigation   2 

Moisture,  effects  in  fumigation  of  citrus  trees   68-69 

Morrill  method  of  measuring  tented  trees  for  dosage   29-30 

Nursery  stock,  fumigation   2 

Opuntia  cngclmanni,  use  of  concoction  for  gas-proofing  fumigating  tents   16 

Oranges,  decay  in  transit,  causes   2-3 

Popper  tree.    (See  Schinus  molle.) 

Poles  for  fumigating  tents   20-21 

Potassium  cyanid.    (See  Cyanid  of  potassium.) 
Pseudococcus  citri.    (See  Mealy-bug.) 
Saissetia  olex.    (See  Scale,  black.) 


112        HYDROCYANIC-ACID  GAS  FUMIGATION  IN  CALIFORNIA. 


Page. 

Scale,  black,  climatic  conditions  most  favorable   76 

enemy  of  citrus  fruits  in  California   7,  8-9 

fumigation   59_60 

dosages  with  potassium  cyanid   59-60 

sodium  cyanid   89 

introduction  of  parasite,  Scutellista  cyanea,  into  California   2 

cottony-cushion.    (See  Icerya  purchasi.) 

insects  affecting  citrus,  effect  of  climatic  conditions  thereon   76-77 

fumigation  dosages  with  sodium  cyanid   89-90 

purple,  climatic  conditions  most  favorable   76 

enemy  of  citrus  fruits  in  California   7,  8 

fumigation,  difficulty  of  destroying  scale  on  fruit   56-57 

dosages  with  potassium  cyanid   55-56 

sodium  cyanid   89 

length  of  exposure   55 

two  successive  treatments   57 

red,  climatic  conditions  most  favorable   76 

enemy  of  citrus  fruits  in  California   7,  9 

fumigation   57-58 

dosages  with  potassium  cyanid   58 

sodium  cyanid   89 

San  Jose,  fumigation  of  trees  therefor   1 

yellow,  climatic  conditions  most  favorable   76 

enemy  of  citrus  fruits  in  California   10 

fumigation   60-61 

dosages  with  potassium  cyanid   60-61 

sodium  cyanid   89 

Schinus  molle,  food  plant  of  black  scale   78 

Scutellista  cyanea,  drawback  to  fumigation  against  black  scale   60 

effectiveness  against  black  scale  on  pepper  tree  (Schinus 

molle)   78 

effect  of  fumigation  thereon  1   77-78 

parasite  of  black  scale  (Saissetia  olex),  introduction  into 

California   2 

Sodium  chlorid,  action  during  generation  of  hydrocyanic-acid  gas   87-88 

effect  of  presence  in  cyanids  on  yield  of  hydrocyanic-acid  gas 

in  fumigations   49-50,  87-88,  99-102 

cyanid.    (See  Cyanid  of  sodium.) 

nitrate,  effect  of  presence  in  cyanids  on  yield  of  hydrocyanic-acid  gas 

in  fumigations   103 

Sooty-mold  fungus.    (See  Fungus,  sooty  mold.) 

Stored  products,  fumigation   2 

Sulphuric  acid,  action  on  hydrocyanic  acid   97-98 

analyses  of  samples  used  for  production  of  hydrocyanic-acid 

gas   91-92 

cost   79 

in  fumigation   41-43 

amount  necessary   42-43 

proportion  with  potassium  cyanid  for  best  yield  of  hydrocyanic- 
acid  gas   93 

sodium  cyanid  for  best  yield  of  hydrocyanic- 
acid  gas   85, 93-96 


INDEX.  113 

Page. 

Supply  cart  for  fumigation,  cost   79 

equipment   22-23 

wagon  for  fumigation,  equipment   23-24 

Syrphus  fly,  enemy  of  mealy-bug  (Pseudococcus  citri)   63 

Tannin  treatment  for  mildew-proofing  fumigating  tents   18-19 

Temperature,  effects  in  fumigation  of  citrus  trees   69-71 

Tents  for  fumigation   10-20 

bell  tents   16 

gas-proofing  them   16-18 

marking  them   19-20 

mildew-proofing  them   IS— 19 

sheet  tents   10-16 

amount  of  cloth  required  for  different  sizes   14-15 

cloth  to  use   12-13 

construction   13-14 

experiments  with  new  tent  material   12 

materials  used   11 

new  tenting  material   11-12 

ring  attachments  and  reen  forcemeats   15-16 

sizes   11 

size  to  purchase   15 

Wagon.    (Set  Supply  wagon.) 

Water  as  a  factor  in  fumigation   44-47 

correct  proportion  with  potassium  cyanid  in  fumigation   47 

sodium  cyanid  in  fumigation   S5 

in  fumigation,  effect  of  different  proportions  on  amount  of  available  gas.  45-47 

temperature  of  gas   45 

proportion  with  potassium  cyanid  for  best  yield  of  hydrocyanic-acid  gas.  93 

sodium  cyanid  for  best  yield  of  hydrocyanic-acid  gas. . .  93-96 

Winds,  effects  in  fumigation  of  citrus  trees   72 

Wogluin,  11.  s.,  paper,  "  Fumigation  of  Citrus  Trees"   1-81 


"The  Value  of  Sodium  Cyanid  for  Fumigation  Purposes"  83-90 


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